Commit Message Guideline

I definitely agree we should try these suggestions and adding structural comments like these in existing reviews makes sense.

The PR body can be changed at any time after the PR is up by editing it in the UI, but this is fine. The PR body should reflect the current state of the PR, including all changes that have been made since it was originally uploaded, so the commit message ends up being the PR title + body whenever it was merged (with the RFC above or if committers take care to do this manually today).