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

從 issue 中連結的 os.path 文件簡介開始，查看詞法函式和相依於主機的函式清單。在進行變更之前，檢查連結的 PR gh-119772。完成的標準是：文件清楚地說明 ntpath 和 posixpath 中哪些函式在其他作業系統上是安全的，以及哪些函式相依於主機作業系統。