Fixed #11441 -- Improved signal topic guide, particularly regarding the weak...

Fixed #11441 -- Improved signal topic guide, particularly regarding the weak and dispatch_uid parameters to the Signal.connect method. Thanks to Mike_A and sayane for the report, and gremmie for the draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14337 bcc190cf-cafb-0310-a4f2-bffc1f526a37