After pointing a new contributor to https://docs.python.org/devguide/gitbootcamp.html#backporting-merged-changes to handle their own backport PRs, I discovered that it's currently taking a step for granted: finding the relevant commit to backport from the CPython master branch, rather than squashing your original PR branch locally.

Since we specifically want to use the squashed-and-merged commit so that the backports include the corresponding commit message, we should make that explicit here.