doc: clarify that CQ only picks up PRs targeting main

PR-URL: #60731
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

Author: Renegade334

doc/contributing/commit-queue.md

@@ -54,6 +54,8 @@ of the commit queue:
 2. A CI must've ran and succeeded since the last change on the PR
 3. A collaborator must have approved the PR since the last change
 4. Only Jenkins CI and GitHub Actions are checked (V8 CI and CITGM are ignored)
+5. The PR must target the `main` branch (PRs opened against other branches, such
+   as backport PRs, are ignored)
 
 ## Implementation