gh-82849: revise intro to os.path.rst (GH-32232)

* revise the first paragraph of docs for os.path
* add a mention of `os.PathLike` protocol
* remove warnings rendered irrelevant by :pep:`383` and :pep:`529`

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
2 files changed