doc: add notable-change to onboarding.md exercise

Pull requests that add new Collaborators should be labeled
`notable-change`. Make a note of this in onboarding.md.

PR-URL: #21040
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/onboarding.md

@@ -211,7 +211,7 @@ needs to be pointed out separately during the onboarding.
     -1`
 * Collaborators are in alphabetical order by GitHub username.
 * Optionally, include your personal pronouns.
-* Label your pull request with the `doc` subsystem label.
+* Label your pull request with the `doc` and `notable-change` labels.
 * Run CI on the PR. Because the PR does not affect any code, use the
   `node-test-pull-request-lite` CI task. Alternatively, use the usual
   `node-test-pull-request` CI task and cancel it after the linter and one other