Add more cross-links about sniff/proxy modes (elastic#112079)

The info about remote cluster connection modes is a little disjointed.
This commit adds some cross-links between the sections to help users
find more relevant information.

docs/reference/modules/cluster/remote-clusters-settings.asciidoc

@@ -6,7 +6,10 @@ mode are described separately.
 
 `cluster.remote.<cluster_alias>.mode`::
   The mode used for a remote cluster connection. The only supported modes are
-  `sniff` and `proxy`.
+  `sniff` and `proxy`. The default is `sniff`. See <<sniff-proxy-modes>> for
+  further information about these modes, and <<remote-cluster-sniff-settings>>
+  and <<remote-cluster-proxy-settings>> for further information about their
+  settings.
 
 `cluster.remote.initial_connect_timeout`::
 
@@ -97,6 +100,11 @@ you configure the remotes.
 [[remote-cluster-sniff-settings]]
 ==== Sniff mode remote cluster settings
 
+To use <<sniff-mode,sniff mode>> to connect to a remote cluster, set
+`cluster.remote.<cluster_alias>.mode: sniff` and then configure the following
+settings. You may also leave `cluster.remote.<cluster_alias>.mode` unset since
+`sniff` is the default mode.
+
 `cluster.remote.<cluster_alias>.seeds`::
 
   The list of seed nodes used to sniff the remote cluster state.
@@ -117,6 +125,10 @@ you configure the remotes.
 [[remote-cluster-proxy-settings]]
 ==== Proxy mode remote cluster settings
 
+To use <<proxy-mode,proxy mode>> to connect to a remote cluster, set
+`cluster.remote.<cluster_alias>.mode: proxy` and then configure the following
+settings.
+
 `cluster.remote.<cluster_alias>.proxy_address`::
 
   The address used for all remote connections.

docs/reference/modules/remote-clusters.asciidoc

@@ -1,7 +1,7 @@
 [[remote-clusters]]
 == Remote clusters
 You can connect a local cluster to other {es} clusters, known as _remote
-clusters_. Remote clusters can be located in different datacenters or 
+clusters_. Remote clusters can be located in different datacenters or
 geographic regions, and contain indices or data streams that can be replicated
 with {ccr} or searched by a local cluster using {ccs}.
 
@@ -30,9 +30,9 @@ capabilities, the local and remote cluster must be on the same
 [discrete]
 === Add remote clusters
 
-NOTE: The instructions that follow describe how to create a remote connection from a 
-self-managed cluster. You can also set up {ccs} and {ccr} from an 
-link:https://www.elastic.co/guide/en/cloud/current/ec-enable-ccs.html[{ess} deployment] 
+NOTE: The instructions that follow describe how to create a remote connection from a
+self-managed cluster. You can also set up {ccs} and {ccr} from an
+link:https://www.elastic.co/guide/en/cloud/current/ec-enable-ccs.html[{ess} deployment]
 or from an link:https://www.elastic.co/guide/en/cloud-enterprise/current/ece-enable-ccs.html[{ece} deployment].
 
 To add remote clusters, you can choose between
@@ -52,7 +52,7 @@ controls. <<remote-clusters-api-key>>.
 
 Certificate based security model::
 Uses mutual TLS authentication for cross-cluster operations. User authentication
-is performed on the local cluster and a user's role names are passed to the 
+is performed on the local cluster and a user's role names are passed to the
 remote cluster. In this model, a superuser on the local cluster gains total read
 access to the remote cluster, so it is only suitable for clusters that are in
 the same security domain. <<remote-clusters-cert>>.
@@ -63,15 +63,17 @@ the same security domain. <<remote-clusters-cert>>.
 
 [[sniff-mode]]
 Sniff mode::
-In sniff mode, a cluster is registered with a name of your choosing and a list
-of addresses of _seed_ nodes. When you register a remote cluster using sniff
-mode, {es} retrieves from one of the seed nodes the addresses of up to three
-_gateway nodes_. Each `remote_cluster_client` node in the local {es} cluster
-then opens several TCP connections to the publish addresses of the gateway
-nodes. This mode therefore requires that the gateway nodes' publish addresses
-are accessible to nodes in the local cluster.
+In sniff mode, a cluster alias is registered with a name of your choosing and a
+list of addresses of _seed_ nodes specified with the
+`cluster.remote.<cluster_alias>.seeds` setting. When you register a remote
+cluster using sniff mode, {es} retrieves from one of the seed nodes the
+addresses of up to three _gateway nodes_. Each `remote_cluster_client` node in
+the local {es} cluster then opens several TCP connections to the publish
+addresses of the gateway nodes. This mode therefore requires that the gateway
+nodes' publish addresses are accessible to nodes in the local cluster.
 +
-Sniff mode is the default connection mode.
+Sniff mode is the default connection mode. See <<remote-cluster-sniff-settings>>
+for more information about configuring sniff mode.
 +
 [[gateway-nodes-selection]]
 The _gateway nodes_ selection depends on the following criteria:
@@ -86,18 +88,23 @@ However, such nodes still have to satisfy the two above requirements.
 
 [[proxy-mode]]
 Proxy mode::
-In proxy mode, a cluster is registered with a name of your choosing and the
-address of a TCP (layer 4) reverse proxy which you must configure to route
-connections to the nodes of the remote cluster. When you register a remote
-cluster using proxy mode, {es} opens several TCP connections to the proxy
-address and uses these connections to communicate with the remote cluster. In
-proxy mode {es} disregards the publish addresses of the remote cluster nodes
-which means that the publish addresses of the remote cluster nodes need not be
-accessible to the local cluster.
+In proxy mode, a cluster alias is registered with a name of your choosing and
+the address of a TCP (layer 4) reverse proxy specified with the
+`cluster.remote.<cluster_alias>.proxy_address` setting. You must configure this
+proxy to route connections to one or more nodes of the remote cluster. When you
+register a remote cluster using proxy mode, {es} opens several TCP connections
+to the proxy address and uses these connections to communicate with the remote
+cluster. In proxy mode {es} disregards the publish addresses of the remote
+cluster nodes which means that the publish addresses of the remote cluster
+nodes need not be accessible to the local cluster.
 +
-Proxy mode is not the default connection mode, so you must configure it
-explicitly if desired. Proxy mode has the same <<gateway-nodes-selection,
-version compatibility requirements>> as sniff mode.
+Proxy mode is not the default connection mode, so you must set
+`cluster.remote.<cluster_alias>.mode: proxy` to use it. See
+<<remote-cluster-proxy-settings>> for more information about configuring proxy
+mode.
++
+Proxy mode has the same <<gateway-nodes-selection, version compatibility
+requirements>> as sniff mode.
 
 include::cluster/remote-clusters-api-key.asciidoc[]