Add instruction how to configure WebSocket extensions to pydoc.

Release note: mod_pywebsocket directory pydoc update. Added note about extension configuration

Review URL: https://codereview.appspot.com/8954043

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@765 4ff78f4a-9131-11de-b045-6380ec9940d4