[squash] add "if they have multiple approvals"

Author: addaleax

COLLABORATOR_GUIDE.md

@@ -65,7 +65,8 @@ from other Collaborators. Leave at least 48 hours during the week and
 and work schedules. Trivial changes (e.g. those which fix minor bugs
 or improve performance without affecting API or causing other
 wide-reaching impact), and focused changes that affect only documentation
-and/or the test suite, may be landed after a shorter delay.
+and/or the test suite, may be landed after a shorter delay if they have
+multiple approvals.
 
 For non-breaking changes, if there is no disagreement amongst
 Collaborators, a pull request may be landed given appropriate review.