Fixed #8325 -- Reorganization and expansion of the login_required decorator...

Fixed #8325 -- Reorganization and expansion of the login_required decorator docs to make it clearer how the redirect_field_name parameter works and improve the overall flow of the text. Thanks to Robert Reeves for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14480 bcc190cf-cafb-0310-a4f2-bffc1f526a37