Adjust resiliency docs for searchable snapshots (#67630)

Today we recommend every index to have at least one replica in our
guidelines for designing a resilient cluster. This advice does not apply
to searchable snapshot indices. This commit adjusts the resiliency docs
to account for this. It also slightly adjusts the wording in the
searchable snapshots docs to be more consistent about the distinction
between a "searchable snapshot" and a "searchable snapshot index".

Author: DaveCTurner

docs/reference/high-availability/cluster-design.asciidoc

@@ -9,16 +9,17 @@ operating normally if some of its nodes are unavailable or disconnected.
 There is a limit to how small a resilient cluster can be. All {es} clusters
 require:
 
-* One <<modules-discovery-quorums,elected master node>> node
-* At least one node for each <<modules-node,role>>.
-* At least one copy of every <<scalability,shard>>.
+- One <<modules-discovery-quorums,elected master node>> node
+- At least one node for each <<modules-node,role>>.
+- At least one copy of every <<scalability,shard>>.
 
 A resilient cluster requires redundancy for every required cluster component.
 This means a resilient cluster must have:
 
-* At least three master-eligible nodes
-* At least two nodes of each role
-* At least two copies of each shard (one primary and one or more replicas)
+- At least three master-eligible nodes
+- At least two nodes of each role
+- At least two copies of each shard (one primary and one or more replicas,
+  unless the index is a <<searchable-snapshots,searchable snapshot index>>)
 
 A resilient cluster needs three master-eligible nodes so that if one of
 them fails then the remaining two still form a majority and can hold a
@@ -44,8 +45,8 @@ failures. Designers of larger clusters must also consider cases where multiple
 nodes fail at the same time. The following pages give some recommendations for
 building resilient clusters of various sizes:
 
-* <<high-availability-cluster-small-clusters>>
-* <<high-availability-cluster-design-large-clusters>>
+- <<high-availability-cluster-small-clusters>>
+- <<high-availability-cluster-design-large-clusters>>
 
 [[high-availability-cluster-small-clusters]]
 === Resilience in small clusters
@@ -78,11 +79,12 @@ one-node clusters in production.
 
 If you have two nodes, we recommend they both be data nodes. You should also
 ensure every shard is stored redundantly on both nodes by setting
-<<dynamic-index-settings,`index.number_of_replicas`>> to `1` on every index.
-This is the default number of replicas but may be overridden by an
-<<index-templates,index template>>. <<dynamic-index-settings,Auto-expand
-replicas>> can also achieve the same thing, but it's not necessary to use this
-feature in such a small cluster.
+<<dynamic-index-settings,`index.number_of_replicas`>> to `1` on every index
+that is not a <<searchable-snapshots,searchable snapshot index>>. This is the
+default behaviour but may be overridden by an <<index-templates,index
+template>>. <<dynamic-index-settings,Auto-expand replicas>> can also achieve
+the same thing, but it's not necessary to use this feature in such a small
+cluster.
 
 We recommend you set `node.master: false` on one of your two nodes so that it is
 not <<master-node,master-eligible>>. This means you can be certain which of your
@@ -162,12 +164,13 @@ cluster that is suitable for production deployments.
 [[high-availability-cluster-design-three-nodes]]
 ==== Three-node clusters
 
-If you have three nodes, we recommend they all be <<data-node,data
-nodes>> and every index should have at least one replica. Nodes are data nodes
-by default. You may prefer for some indices to have two replicas so that each
-node has a copy of each shard in those indices. You should also configure each
-node to be <<master-node,master-eligible>> so that any two of them can hold a
-master election without needing to communicate with the third node. Nodes are
+If you have three nodes, we recommend they all be <<data-node,data nodes>> and
+every index that is not a <<searchable-snapshots,searchable snapshot index>>
+should have at least one replica. Nodes are data nodes by default. You may
+prefer for some indices to have two replicas so that each node has a copy of
+each shard in those indices. You should also configure each node to be
+<<master-node,master-eligible>> so that any two of them can hold a master
+election without needing to communicate with the third node. Nodes are
 master-eligible by default. This cluster will be resilient to the loss of any
 single node.
 
@@ -215,8 +218,8 @@ The cluster will be resilient to the loss of any node as long as:
 
 - The <<cluster-health,cluster health status>> is `green`.
 - There are at least two data nodes. 
-- Every index has at least one replica of each shard, in addition to the 
-  primary.
+- Every index that is not a <<searchable-snapshots,searchable snapshot index>>
+  has at least one replica of each shard, in addition to the primary.
 - The cluster has at least three master-eligible nodes, as long as at least two
   of these nodes are not voting-only master-eligible nodes.
 - Clients are configured to send their requests to more than one node or are
@@ -326,14 +329,14 @@ zone. If you have more than three zones then you should choose three of the
 zones and put a master-eligible node in each of these three zones. This will
 mean that the cluster can still elect a master even if one of the zones fails.
 
-As always, your indices should have at least one replica in case a node fails.
-You should also use <<allocation-awareness,shard allocation awareness>> to
-limit the number of copies of each shard in each zone. For instance, if you have
-an index with one or two replicas configured then allocation awareness will
-ensure that the replicas of the shard are in a different zone from the primary.
-This means that a copy of every shard will still be available if one zone
-fails. The availability of this shard will not be affected by such a
-failure.
+As always, your indices should have at least one replica in case a node fails,
+unless they are <<searchable-snapshots,searchable snapshot indices>>. You
+should also use <<allocation-awareness,shard allocation awareness>> to limit
+the number of copies of each shard in each zone. For instance, if you have an
+index with one or two replicas configured then allocation awareness will ensure
+that the replicas of the shard are in a different zone from the primary. This
+means that a copy of every shard will still be available if one zone fails. The
+availability of this shard will not be affected by such a failure.
 
 [[high-availability-cluster-design-large-cluster-summary]]
 ==== Summary
@@ -342,8 +345,8 @@ The cluster will be resilient to the loss of any zone as long as:
 
 - The <<cluster-health,cluster health status>> is `green`.
 - There are at least two zones containing data nodes.
-- Every index has at least one replica of each shard, in addition to the 
-  primary.
+- Every index that is not a <<searchable-snapshots,searchable snapshot index>>
+  has at least one replica of each shard, in addition to the primary.
 - Shard allocation awareness is configured to avoid concentrating all copies of
   a shard within a single zone.
 - The cluster has at least three master-eligible nodes. At least two of these 

docs/reference/searchable-snapshots/index.asciidoc

@@ -36,20 +36,22 @@ index setting.
 If a node fails and {search-snap} shards need to be restored from the snapshot,
 there is a brief window of time while {es} allocates the shards to other nodes
 where the cluster health will not be `green`. Searches that hit these shards
-will fail or return partial results until they are reallocated.
+will fail or return partial results until the shards are reallocated to healthy
+nodes.
 
 You typically manage {search-snaps} through {ilm-init}. The
 <<ilm-searchable-snapshot, searchable snapshots>> action automatically converts
-an index to a {search-snap} when it reaches the `cold` phase. You can also make
-indices in existing snapshots searchable by manually mounting them as
-{search-snaps} with the <<searchable-snapshots-api-mount-snapshot, mount
-snapshot>> API.
+a regular index into a {search-snap} index when it reaches the `cold` phase.
+You can also make indices in existing snapshots searchable by manually mounting
+them as {search-snap} indices with the
+<<searchable-snapshots-api-mount-snapshot, mount snapshot>> API.
 
 To mount an index from a snapshot that contains multiple indices, we recommend
 creating a <<clone-snapshot-api, clone>> of the snapshot that contains only the
-index you want to search, and mounting the clone. You cannot delete a snapshot
-if it has any mounted indices, so creating a clone enables you to manage the
-lifecycle of the backup snapshot independently of any {search-snaps}.
+index you want to search, and mounting the clone. You should not delete a
+snapshot if it has any mounted indices, so creating a clone enables you to
+manage the lifecycle of the backup snapshot independently of any
+{search-snaps}.
 
 You can control the allocation of the shards of {search-snap} indices using the
 same mechanisms as for regular indices. For example, you could use