[DOCS] Clarify connections and gateway nodes selection in cross cluster search docs (#24859)

Closes #24836

Author: javanna

docs/reference/modules/cross-cluster-search.asciidoc

@@ -10,7 +10,12 @@ federated search requests.
 
 Cross cluster search works by configuring a remote cluster in the cluster state and connecting only to a
 limited number of nodes in the remote cluster. Each remote cluster is referenced by a name and a list of seed nodes.
-Those seed nodes are used to discover nodes in the remote cluster which are eligible as _gateway nodes_.
+When a remote cluster is registered, its cluster state is retrieved from one of the seed nodes so that up to 3
+_gateway nodes_ are selected to be connected to as part of upcoming cross cluster search requests.
+Cross cluster search requests consist of uni-directional connections from the coordinating node to the previously
+selected remote nodes only. It is possible to tag which nodes should be selected through
+node attributes (see <<cross-cluster-search-settings>>).
+
 Each node in a cluster that has remote clusters configured connects to one or more _gateway nodes_ and uses
 them to federate search requests to the remote cluster.
 
@@ -169,6 +174,7 @@ will be prefixed with their remote cluster name:
 // TESTRESPONSE
 
 [float]
+[[cross-cluster-search-settings]]
 === Cross cluster search settings
 
 `search.remote.connections_per_cluster`::