bpo-33725, multiprocessing doc: rephase warning against fork on macOS (GH-13841)

Co-Authored-By: Barry Warsaw <[email protected]>

Author: vstinner

Doc/library/multiprocessing.rst

@@ -126,8 +126,9 @@ to start a process.  These *start methods* are
 
 .. versionchanged:: 3.8
 
-   On macOS, *spawn* start method is now the default: *fork* start method is no
-   longer reliable on macOS, see :issue:`33725`.
+   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`.
 
 .. versionchanged:: 3.4
    *spawn* added on all unix platforms, and *forkserver* added for