Clarified release note about the removal of 'mixin' parameter to Storage.open,...

Clarified release note about the removal of 'mixin' parameter to Storage.open, and added a note in the deprecation timeline.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17161 bcc190cf-cafb-0310-a4f2-bffc1f526a37