doc: link cluster worker.send() to child.send()

PR-URL: #839
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>

doc/api/cluster.markdown

@@ -420,11 +420,13 @@ exit, the master may choose not to respawn a worker based on this value.
 * `message` {Object}
 * `sendHandle` {Handle object}
 
-This function is equal to the send methods provided by
-`child_process.fork()`.  In the master you should use this function to
-send a message to a specific worker.
+Send a message to a worker or master, optionally with a handle.
 
-In a worker you can also use `process.send(message)`, it is the same function.
+In the master this sends a message to a specific worker. It is identical to
+[child.send()](child_process.html#child_process_child_send_message_sendhandle).
+
+In a worker this sends a message to the master. It is identical to
+`process.send()`.
 
 This example will echo back all messages from the master: