Some ntpath/posixpath functions unusable on foreign OS

The os.path docs intro includes:

Note: Since different operating systems have different path name conventions, there are several versions of this module in the standard library. The os.path module is always the path module suitable for the operating system Python is running on, and therefore usable for local paths. However, you can also import and use the individual modules if you want to manipulate a path that is always in one of the different formats. They all have the same interface:

• posixpath for UNIX-style paths
• ntpath for Windows paths

But we don't explain that some of the functions in ntpath and posixpath aren't suitable for usage on a "foreign" OS, e.g. that using ntpath.realpath() from POSIX is a bad idea.

These functions are safe because they do purely lexical work:

• basename
• commonpath
• commonprefix
• dirname
• isabs
• join
• normcase
• normpath
• split
• splitdrive
• splitroot
• splitext

These functions are probably unsafe because they rely on details of the host OS (e.g. via the os module):

• abspath
• exists
• lexists
• expanduser
• expandvars
• getatime
• getmtime
• getctime
• getsize
• isfile
• isdir
• isjunction
• islink
• ismount
• isdevdrive
• realpath
• relpath
• samefile
• sameopenfile
• samestat
• supports_unicode_filenames

Linked PRs

• gh-119772

Start with the os.path documentation intro linked in the issue and review the lexical and host-dependent function lists. Check linked PR gh-119772 before making changes. Done means the documentation clearly explains which ntpath and posixpath functions are safe on a foreign OS and which depend on the host OS.