doc: update onboarding task

Use fast-tracking on PRs where new Collaborators are adding themselves
to the README.

PR-URL: nodejs#23300
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/onboarding.md

@@ -215,8 +215,8 @@ needs to be pointed out separately during the onboarding.
   labels.
 * Run CI on the PR. Because the PR does not affect any code, use the
   `node-test-pull-request-lite-pipeline` CI task.
-* After one or two approvals, land the PR (PRs of this type do not need to wait
-  for 48 hours to land).
+* After two Collaborator approvals for the change and two Collaborator approvals
+  for fast-tracking, land the PR.
   * Be sure to add the `PR-URL: <full-pr-url>` and appropriate `Reviewed-By:`
     metadata.
   * [`node-core-utils`][] automates the generation of metadata and the landing