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>

doc/api/child_process.md

@@ -572,9 +572,9 @@ spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
 *It is worth noting that when an IPC channel is established between the
 parent and child processes, and the child is a Node.js process, the child
 is launched with the IPC channel unreferenced (using `unref()`) until the
-child registers an event handler for the [`process.on('disconnect')`][] event.
+child registers an event handler for the [`process.on('disconnect')`][] event
-This allows the child to exit normally without the process being held open
+or the [`process.on('message')`][] event.This allows the child to exit normally
-by the open IPC channel.*
+without the process being held open by the open IPC channel.*
 
 See also: [`child_process.exec()`][] and [`child_process.fork()`][]