Fixed #12691 -- Added a versionadded tag to the description of widget...

Fixed #12691 -- Added a versionadded tag to the description of widget overrides in forms. Thanks to danielr for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13108 bcc190cf-cafb-0310-a4f2-bffc1f526a37