Issue #5501: Clarify that invoking freeze_support() on non-Windows platforms has no effect

Patch by Davin Potts and Camilla Montonen.

Doc/library/multiprocessing.rst

@@ -906,8 +906,10 @@ Miscellaneous
    If the ``freeze_support()`` line is omitted then trying to run the frozen
    executable will raise :exc:`RuntimeError`.
 
-   If the module is being run normally by the Python interpreter then
-   :func:`freeze_support` has no effect.
+   Calling ``freeze_support()`` has no effect when invoked on any operating
+   system other than Windows.  In addition, if the module is being run
+   normally by the Python interpreter on Windows (the program has not been
+   frozen), then ``freeze_support()`` has no effect.
 
 .. function:: get_all_start_methods()