[docs] Update documentation for multiprocessing.get_start_method (GH-18170) (GH-28535)

(cherry picked from commit af90b54)

Co-authored-by: Sam Sneddon <[email protected]>

Author: miss-islington

Doc/library/multiprocessing.rst

@@ -1019,7 +1019,13 @@ Miscellaneous
 
    The return value can be ``'fork'``, ``'spawn'``, ``'forkserver'``
    or ``None``.  ``'fork'`` is the default on Unix, while ``'spawn'`` is
-   the default on Windows.
+   the default on Windows and macOS.
+
+.. versionchanged:: 3.8
+
+   On macOS, the *spawn* start method is now the default.  The *fork* start
+   method should be considered unsafe as it can lead to crashes of the
+   subprocess. See :issue:`33725`.
 
    .. versionadded:: 3.4