when landed remove the
"need backport tags" you added...

If done correctly, the "needs backport .." labels got removed automatically. We have detailed info here: https://devguide.python.org/committing/#backporting-changes-to-an-older-version

a) is it ok to touch 3.9, as it's in rc1?

Yeah bug fixes are accepted to the maintenance branches. I think your PR does count as documentation bug fix, so it should be ok to backport to 3.9

b) shall I create two brand new branches for those PRs, or there's
some way in github/git to "just propose this same change to the other
branches"?

In most cases the bot can handle it simply by adding the "needs backport " label. But for more complicated situations, please use the cherry_picker.py command line tool.

The documentation on how to do it is here: https://github.com/python/cherry-picker#cherry-picking