doc: update onboarding.md for faster exercise completion

Note that the CI run for the exercise can be minimal.

PR-URL: #18846
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: Trott

doc/onboarding.md

@@ -200,7 +200,10 @@ onboarding session.
 * Collaborators are in alphabetical order by GitHub username.
 * Optionally, include your personal pronouns.
 * Label your pull request with the `doc` subsystem label.
-* Run CI on your PR.
+* 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
+  subtask have passed.
 * After one or two approvals, land the PR (PRs of this type do not need to wait
   for 48/72 hours to land).
   * Be sure to add the `PR-URL: <full-pr-url>` and appropriate `Reviewed-By:`