Clarify usage of type hints in the standard-library modules

[Privat33r-dev]

I think that Alex summarized the issue pretty well:

In general our policy is currently not to add type hints to standard-library modules in the Lib/ directory (there are some exceptions, but they're all pretty specific). I think we still have yet to add some formal documentation around this in the devguide (we really need to!), but you can see some rationales here:

• GH-83417: Allow venv to add a .gitignore file to environments via a new scm_ignore_file parameter cpython#108125 (comment)
• https://discuss.python.org/t/type-annotations-in-the-stdlib/21487

Originally posted by @AlexWaygood in python/cpython#117583 (comment)