doc: specify types of listener parameter

We cannot manage with `{any}` because `message` cannot be
of `Symbol` or `undefined` types.

PR-URL: #20405
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/process.md

@@ -84,7 +84,8 @@ process.on('exit', (code) => {
 added: v0.5.10
 -->
 
-* `message` {Object} a parsed JSON object or primitive value.
+* `message` { Object | boolean | number | string | null } a parsed JSON object
+  or a serializable primitive value.
 * `sendHandle` {net.Server|net.Socket} a [`net.Server`][] or [`net.Socket`][]
   object, or undefined.