[DOCS] Add 'Selecting gateway and seed nodes' section to CCS docs (#48297)

Author: jrodewig

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

@@ -246,9 +246,31 @@ PUT _cluster/settings
 If `cluster_two` is disconnected or unavailable during a {ccs}, {es} won't
 include matching documents from that cluster in the final results.
 
-[float]
+[discrete]
 [[ccs-works]]
 == How {ccs} works
+
+include::./remote-clusters.asciidoc[tag=how-remote-clusters-work]
+
+[discrete]
+[[ccs-gateway-seed-nodes]]
+=== Selecting gateway and seed nodes
+
+Gateway and seed nodes need to be accessible from the local cluster via your
+network.
+
+By default, any master-ineligible node can act as a gateway node. If wanted,
+you can define the gateway nodes for a cluster by setting
+`cluster.remote.node.attr.gateway` to `true`.
+
+For {ccs}, we recommend you use gateway nodes that are capable of serving as
+<<coordinating-node,coordinating nodes>> for search requests. If
+wanted, the seed nodes for a cluster can be a subset of these gateway nodes.
+
+[discrete]
+[[ccs-network-delays]]
+=== How {ccs} handles network delays
+
 Because {ccs} involves sending requests to remote clusters, any network delays
 can impact search speed. To avoid slow searches, {ccs} offers two options for
 handling network delays:
@@ -272,11 +294,9 @@ latency.
 +
 See <<ccs-unmin-roundtrips>> to learn how this option works.
 
-
-
 [float]
 [[ccs-min-roundtrips]]
-=== Minimize network roundtrips
+==== Minimize network roundtrips
 
 Here's how {ccs} works when you minimize network roundtrips.
 
@@ -301,7 +321,7 @@ image:images/ccs/ccs-min-roundtrip-client-response.png[]
 
 [float]
 [[ccs-unmin-roundtrips]]
-=== Don't minimize network roundtrips
+==== Don't minimize network roundtrips
 
 Here's how {ccs} works when you don't minimize network roundtrips.
 

docs/reference/modules/remote-clusters.asciidoc

@@ -13,12 +13,16 @@ connections to a remote cluster. This functionality is used in
 <<modules-cross-cluster-search,{ccs}>>.
 endif::[]
 
+// tag::how-remote-clusters-work[]
 Remote cluster connections work by configuring a remote cluster and connecting
 only to a limited number of nodes in that remote cluster. Each remote cluster
 is referenced by a name and a list of seed nodes. When a remote cluster is
 registered, its cluster state is retrieved from one of the seed nodes and up
 to three _gateway nodes_ are selected to be connected to as part of remote
-cluster requests. All the communication required between different clusters
+cluster requests.
+// end::how-remote-clusters-work[]
+
+All the communication required between different clusters
 goes through the <<modules-transport,transport layer>>. Remote cluster
 connections consist of uni-directional connections from the coordinating
 node to the selected remote _gateway nodes_ only.