docs: clairfy advertise.rpc effect

The advertise.rpc config option is not intuitive. At first glance you'd
assume it works like advertise.http or advertise.serf, but it does not.

The current behavior is working as intended, but the documentation is
very hard to parse and doesn't draw a clear picture of what the setting
actually does.

Closes #11075

website/content/docs/configuration/index.mdx

@@ -116,9 +116,12 @@ testing.
     reachable by all the nodes from which end users are going to use the Nomad
     CLI tools.
 
-  - `rpc` - The address advertised to Nomad client nodes. This allows
-    advertising a different RPC address than is used by Nomad Servers such that
-    the clients can connect to the Nomad servers if they are behind a NAT.
+  - `rpc` - The address used to advertise to Nomad clients for connecting to Nomad
+    servers for RPC. This allows Nomad clients to connect to Nomad servers from
+    behind a NAT gateway. This address much be reachable by all Nomad client nodes.
+    When set, the Nomad servers will use the `advertise.serf` address for RPC
+    connections amongst themselves. Setting this value on a Nomad client has no
+    effect.
 
   - `serf` - The address advertised for the gossip layer. This address must be
     reachable from all server nodes. It is not required that clients can reach