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...!
> ... it is unusual that a user would expect to be able to "import foo" after installing an application.

Maybe "it is unusual that a user would expect to be able to "import foo" after installing the `foo` application."