Document supported scenarios for CCS (backport of #58120) (#59894)

Documents the supported scenarios for CCS.

Co-authored-by: Adam Locke <adam.locke@elastic.co>

Author: nik9000

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

@@ -376,3 +376,21 @@ image:images/ccs/ccs-dont-min-roundtrip-shard-results.svg[]
 final results in the {ccs} response.
 +
 image:images/ccs/ccs-min-roundtrip-client-response.svg[]
+
+[discrete]
+[[ccs-supported-configurations]]
+=== Supported configurations
+
+Generally, <<gateway-nodes-selection, cross cluster search>> can search remote
+clusters that are one major version ahead or behind the coordinating node's
+version. Cross cluster search can also search remote clusters that are being
+<<rolling-upgrades, upgraded>> so long as both the "upgrade from" and
+"upgrade to" version are compatible with the gateway node.
+
+For example, a coordinating node running {es} 5.6 can search a remote cluster
+running {es} 6.8, but that cluster can not be upgraded to 7.1. In this case
+you should first upgrade the coordinating node to 7.1 and then upgrade remote
+cluster.
+
+WARNING: Running multiple versions of {es} in the same cluster beyond the
+duration of an upgrade is not supported.