doc: add missing events.on metadata

This was missed in the original PR.

Refs: #34912

PR-URL: #37965
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

doc/api/events.md

@@ -849,6 +849,10 @@ const { getEventListeners, EventEmitter } = require('events');
 added:
  - v11.13.0
  - v10.16.0
+changes:
+  - version: v15.0.0
+    pr-url: https://github.com/nodejs/node/pull/34912
+    description: The `signal` option is supported now.
 -->
 
 * `emitter` {EventEmitter}