Fixed #14307 -- Linked ChoiceField.choices docs to Field.choices docs to...

Fixed #14307 -- Linked ChoiceField.choices docs to Field.choices docs to explain the formatting options available. Thanks to adamv for the report and Russ for the suggested fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14231 bcc190cf-cafb-0310-a4f2-bffc1f526a37