Issue #23254: Document how to close the TCPServer listening socket.

Patch from Martin Panter.
diff --git a/Lib/test/test_socketserver.py b/Lib/test/test_socketserver.py
index 8707017..714ca4a 100644
--- a/Lib/test/test_socketserver.py
+++ b/Lib/test/test_socketserver.py
@@ -158,6 +158,8 @@
         if verbose: print "waiting for server"
         server.shutdown()
         t.join()
+        server.server_close()
+        self.assertRaises(socket.error, server.socket.fileno)
         if verbose: print "done"
 
     def stream_examine(self, proto, addr):