doc: note message event listeners ref IPC channels

The IPC channel is referenced with the message event too.

PR-URL: https://github.com/nodejs/node/pull/11494


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>