Fixed #12800 -- Clarified the text describing the palettes available to...

Fixed #12800 -- Clarified the text describing the palettes available to django-admin.py, and when they will be used. Thanks to rasmus and Karen Tracey for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12517 bcc190cf-cafb-0310-a4f2-bffc1f526a37