doc: fix cluster message event docs

Fixes: https://github.com/nodejs/node/issues/7997
PR-URL: https://github.com/nodejs/node/pull/8017
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/cluster.md

@@ -488,7 +488,7 @@ The `addressType` is one of:
 * `message` {Object}
 * `handle` {undefined|Object}
 
-Emitted when any worker receives a message.
+Emitted when the cluster master receives a message from any worker.
 
 See [child_process event: 'message'][].