meta: clarify fast-track approval

Refs: #23725 (comment)

PR-URL: #23744
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

COLLABORATOR_GUIDE.md

@@ -187,7 +187,9 @@ Collaborators that previously approved the pull request. If someone disagrees
 with the fast-tracking request, remove the label and leave a comment indicating
 why the pull request should not be fast-tracked. The pull request can be landed
 once two or more Collaborators approve both the pull request and the
-fast-tracking request, and the necessary CI testing is done.
+fast-tracking request, and the necessary CI testing is done. A request to
+fast-track a PR made by a different Collaborator than the pull-request author
+counts as a fast-track approval.
 
 ### Testing and CI