[docs] Update documentation for `multiprocessing.get_start_method` (GH-18170) (GH-28533)

(cherry picked from commit af90b5498b8c6acd67b50fdad007d26dfd1c5823)

Co-authored-by: Sam Sneddon <me@gsnedders.com>
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 3d78ad3..7225b65 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -1030,7 +1030,13 @@
 
    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