authors cannot approve their own pull requests (github#19116)

Author: chiph

data/reusables/repositories/request-changes-tips.md

@@ -5,5 +5,6 @@
 - Repository owners and administrators can merge a pull request even if it hasn't received an approving review, or if a reviewer who requested changes has left the organization or is unavailable.
 - If both required reviews and stale review dismissal are enabled and a code-modifying commit is pushed to the branch of an approved pull request, the approval is dismissed. The pull request must be reviewed and approved again before it can be merged.
 - When several open pull requests each have a head branch pointing to the same commit, you won’t be able to merge them if one or both have a pending or rejected review.
+- Pull request authors cannot approve their own pull requests.
 
 {% endtip %}