Add note about ILM and Snapshots (#36023)

This commit documents how Index Lifecycle Management
interacts with snapshot/restore, and documents a workaround
for situations in which ILM should not immediately resume
managing an index after it is restored.

Author: gwbrown

docs/reference/ilm/ilm-and-snapshots.asciidoc

@@ -0,0 +1,35 @@
+[role="xpack"]
+[testenv="basic"]
+[[index-lifecycle-and-snapshots]]
+== Restoring Snapshots of Managed Indices
+
+beta[]
+
+When restoring a snapshot that contains indices managed by Index Lifecycle
+Management, the lifecycle will automatically continue to execute after the
+snapshot is restored. Notably, the `min_age` is relative to the original
+creation or rollover of the index, rather than when the index was restored. For
+example, a monthly index that is restored partway through its lifecycle after an
+accidental deletion will be continue through its lifecycle as expected: The
+index will be shrunk, reallocated to different nodes, or deleted on the same
+schedule whether or not it has been restored from a snapshot.
+
+However, there may be cases where you need to restore an index from a snapshot,
+but do not want it to automatically continue through its lifecycle, particularly
+if the index would rapidly progress through lifecycle phases due to its age. For
+example, you may wish to add or update documents in an index before it is marked
+read only or shrunk, or prevent an index from automatically being deleted.
+
+To stop lifecycle policy execution on an index restored from a snapshot, before
+restoring the snapshot, <<start-stop-ilm,lifecycle policy execution can be
+paused>> to allow the policy to be removed.
+
+For example, the following workflow can be used in the above situation to
+prevent the execution of the lifecycle policy for an index:
+
+1. Pause execution of all lifecycle policies using the <<ilm-stop,Stop ILM API>>
+2. Restore the snapshot.
+3. Perform whatever operations you wish before resuming lifecycle execution, or
+    remove the lifecycle policy from the index using the
+    <<ilm-remove-policy,Remove Policy from Index API>>
+4. Resume execution of lifecycle policies using the <<ilm-start,Start ILM API>>

docs/reference/ilm/index.asciidoc

@@ -64,4 +64,6 @@ include::update-lifecycle-policy.asciidoc[]
 
 include::error-handling.asciidoc[]
 
+include::ilm-and-snapshots.asciidoc[]
+
 include::start-stop-ilm.asciidoc[]