Fixed #14512 -- Added documentation on how to apply decorators to class-based...

Fixed #14512 -- Added documentation on how to apply decorators to class-based generic views. Thanks to Łukasz Rekucki for his work on the issue.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14642 bcc190cf-cafb-0310-a4f2-bffc1f526a37