bpo-39797 Changes to socketserver.BaseServer's shutdown() method. (GH-18929)



Automerge-Triggered-By: @ned-deily
diff --git a/Doc/library/socketserver.rst b/Doc/library/socketserver.rst
index 7c8c8d5..232c061 100644
--- a/Doc/library/socketserver.rst
+++ b/Doc/library/socketserver.rst
@@ -237,6 +237,8 @@
    .. method:: shutdown()
 
       Tell the :meth:`serve_forever` loop to stop and wait until it does.
+      :meth:`shutdown` must be called while :meth:`serve_forever` is running in a
+      different thread otherwise it will deadlock.
 
 
    .. method:: server_close()