improve os.mknod() documentation

[calestyo]

Documentation

Split out from #107924:

cpython/Doc/library/os.rst

Lines 2398 to 2417 in cc2cf85

	.. function:: mknod(path, mode=0o600, device=0, *, dir_fd=None)
	Create a filesystem node (file, device special file or named pipe) named
	*path*. *mode* specifies both the permissions to use and the type of node
	to be created, being combined (bitwise OR) with one of ``stat.S_IFREG``,
	``stat.S_IFCHR``, ``stat.S_IFBLK``, and ``stat.S_IFIFO`` (those constants are
	available in :mod:`stat`). For ``stat.S_IFCHR`` and ``stat.S_IFBLK``,
	*device* defines the newly created device special file (probably using
	:func:`os.makedev`), otherwise it is ignored.
	This function can also support :ref:`paths relative to directory descriptors
	<dir_fd>`.
	.. availability:: Unix, not Emscripten, not WASI.
	.. versionadded:: 3.3
	The *dir_fd* argument.
	.. versionchanged:: 3.6
	Accepts a :term:`path-like object`.

mknod(), just as its POSIX counterpart, seems to work just fine with S_IFSOCK as well.

However, POSIX documents any but S_IFIFO as being “non-portable” and mkfifo() as being preferred when creating FIFOs.

I think all these things should somehow be reflected.