docs: Python for formula authors

Begin to document the current consensus about correct Python behavior in Homebrew formulas. The libraries advice is a little more experimental than the applications advice but I've started to experiment with it in homebrew-python and nobody's complained yet...!
Yes, I agree. Also, part of the reason for my not wanting to support lots of different configurations is because it's error-prone, is a lot of work for a very small proportion of users and we should focus on sensible defaults over adding more customisation. I defer to @tdsmith when it comes to Python decisions as he's done far more work recently and uses it (and thus cares about it) far more than I do.