doc: detail CI sub-tasks rerunning

PR-URL: #20200
Fixes: #20192
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

COLLABORATOR_GUIDE.md

@@ -199,11 +199,14 @@ status indicator.
 
 Do not land any Pull Requests without passing (green or yellow) CI runs. If you
 believe any failed (red or grey) CI sub-tasks are unrelated to the change in the
-Pull Request, you may re-run the sub-task to try to see if it passes. If re-runs
-of all failed sub-tasks pass, it is permissible to land the Pull Request but
-only if the initial failures are believed in good faith to be unrelated to the
-changes in the Pull Request. Otherwise, reasonable steps must be taken to
-confirm that the changes are not resulting in an unreliable test.
+Pull Request, you may re-run the sub-task to try to see if it passes (just open
+the failed sub-task page and press the "Rebuild" button; be sure you are still
+logged in for this action). If re-runs of all failed sub-tasks pass (do not
+forget to provide the links for successfully rerun sub-tasks), it is permissible
+to land the Pull Request but only if the initial failures are believed in good
+faith to be unrelated to the changes in the Pull Request. Otherwise, reasonable
+steps must be taken to confirm that the changes are not resulting in an
+unreliable test.
 
 #### Useful CI Jobs