Add documentation for transport.publish_port

docs/reference/modules/transport.asciidoc

@@ -25,6 +25,12 @@ TCP. It allows for the following settings:
 |Setting |Description
 |`transport.tcp.port` |A bind port range. Defaults to `9300-9400`.
 
+|`transport.publish_port` |The port that other nodes in the cluster
+should use when communicating with this node. Useful when a cluster node
+is behind a proxy or firewall and the `transport.tcp.port` is not directly
+addressable from the outside. Defaults to the actual port assigned via
+`transport.tcp.port`.
+
 |`transport.tcp.connect_timeout` |The socket connect timeout setting (in
 time setting format). Defaults to `2s`.