bpo-9811: [doc] strftime handling of unsupported format specifiers is platform dependent (GH-28264)

Author: iritkatriel

Doc/library/datetime.rst

@@ -2431,7 +2431,8 @@ incomplete or ambiguous ISO 8601 directives will raise a :exc:`ValueError`.
 The full set of format codes supported varies across platforms, because Python
 calls the platform C library's :func:`strftime` function, and platform
 variations are common. To see the full set of format codes supported on your
-platform, consult the :manpage:`strftime(3)` documentation.
+platform, consult the :manpage:`strftime(3)` documentation. There are also
+differences between platforms in handling of unsupported format specifiers.
 
 .. versionadded:: 3.6
    ``%G``, ``%u`` and ``%V`` were added.