[docs] Document how to close the TCPServer listening socket (issue 23254)

[demianbrecht at gmail.com]

http://bugs.python.org/review/23254/diff/13794/Doc/library/socketserver.rst
File Doc/library/socketserver.rst (right):

http://bugs.python.org/review/23254/diff/13794/Doc/library/socketserver.rst#newcode169
Doc/library/socketserver.rst:169: Clean up the server.  May be
overridden.
The extra space here should be removed.

http://bugs.python.org/review/23254/diff/13794/Lib/test/test_socketserver.py
File Lib/test/test_socketserver.py (right):

http://bugs.python.org/review/23254/diff/13794/Lib/test/test_socketserver.py#newcode148
Lib/test/test_socketserver.py:148:
self.assertTrue(server.socket._closed)
Small nitpick: Rather than relying on internals which can change, this
could be:

self.assertEqual(server.socket.fileno(), -1)

http://bugs.python.org/review/23254/