Add additional pointers to pathlib's mapping to os.path functions (GH-94828)

* Add additional pointers to pathlib's mapping to os.path functions

os.path.splitext has a somewhat quirky signature since it mixes the path and filename components but I wanted the documentation to mention `PurePath.stem` as the natural counterpart to `PurePath.suffix` for the common use of `os.path.splitext` to turn "file.py" into "file" and "py".

Technically this could have some discussion of how to handle the parent directory hierarchy but that seems a bit out of keeping with the spirit of this table so I omitted mentioning `PurePath.parents` here.

* Update Doc/library/pathlib.rst

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>
(cherry picked from commit 3789c635772fbdeb5d0fc32aa811fd6b1d935a60)

Co-authored-by: Chris Adams <chris@improbable.org>