bpo-31233, bpo-31151: Document socketserver changes (#5417)

socketserver.ForkingMixIn.server_close() and
socketserver.ThreadingMixIn.server_close() now waits until all child
processes and non-daemonic threads complete.
diff --git a/Doc/library/socketserver.rst b/Doc/library/socketserver.rst
index e12c8c9..5600027 100644
--- a/Doc/library/socketserver.rst
+++ b/Doc/library/socketserver.rst
@@ -115,6 +115,21 @@
    :class:`ForkingMixIn` and the Forking classes mentioned below are
    only available on POSIX platforms that support :func:`~os.fork`.
 
+   :meth:`socketserver.ForkingMixIn.server_close` waits until all child
+   processes complete.
+
+   :meth:`socketserver.ThreadingMixIn.server_close` waits until all non-daemon
+   threads complete. Use daemonic threads by setting
+   :data:`ThreadingMixIn.daemon_threads` to ``True`` to not wait until threads
+   complete.
+
+   .. versionchanged:: 3.7
+
+      :meth:`socketserver.ForkingMixIn.server_close` and
+      :meth:`socketserver.ThreadingMixIn.server_close` now waits until all
+      child processes and non-daemonic threads complete.
+
+
 .. class:: ForkingTCPServer
            ForkingUDPServer
            ThreadingTCPServer