Match docstring and args for posixpath.join and ntpath.join

[brl0]

Adpated docstring from posixpath.join to ntpath.join, which was previously blank.

Also renamed arguments to posixpath.join to match ntpath.join. This is a cosmetic change, but I think they should have matching signatures since they are used interchangeably by os.path and pathlib.

Here are the current signatures:

function	signature
posixpath.join	def join(a, *p)
ntpath.join	def join(path, *paths)

This change is not much more than fixing a typo, so I didn't open an issue for this, but would be willing to do so if that is preferred.
Also, this change is probably not news-worthy, but I can add an entry if needed.

[ghost]

All commit authors signed the Contributor License Agreement.

[barneygale]

In ntpath, "absolute path" means a path with a root, such as "\bar". If this path does not have a drive letter, then the previous drive letter (if any) is retained. So it's not right to say that all previous components will be discarded -- it's a little more subtle than that!

[brl0]

Thanks for the clarification! I made a change to be more descriptive, let me know if it still needs work.

[barneygale]

Adding a docstring for os.path.join() on Windows is worthy of an issue and a news entry, IMO.

[brl0]

@barneygale,

Thanks for your quick review and feedback, it is much appreciated. I have made the requested changes; please review again.

Also, thanks for your great work on pathlib. I have been contributing to a project universal_pathlib, based on fsspec, and it was in that context I was digging around some of your changes in cpython to begin with. I was trying to figure out what would need to be adapted to properly implement a new path flavour module for classes inheriting from pathlib, which is why I was looking closely at docstrings and function args.

[arhadthedev]

Do we need a news entry for documentation?