bpo-42317: Improve docs of typing.get_args concerning Union (GH-23254)

(cherry picked from commit c3b9592)

Co-authored-by: Dominik1123 <15989985+Dominik1123@users.noreply.github.com>

Doc/library/typing.rst

@@ -1687,6 +1687,9 @@ Introspection helpers
    For a typing object of the form ``X[Y, Z, ...]`` these functions return
    ``X`` and ``(Y, Z, ...)``. If ``X`` is a generic alias for a builtin or
    :mod:`collections` class, it gets normalized to the original class.
+   If ``X`` is a :class:`Union` contained in another generic type,
+   the order of ``(Y, Z, ...)`` may be different from the order of
+   the original arguments ``[Y, Z, ...]`` due to type caching.
    For unsupported objects return ``None`` and ``()`` correspondingly.
    Examples::