[1.2.X] Fixed #14307 -- Linked ChoiceField.choices docs to Field.choices docs...

[1.2.X] 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.

Backport of [14231] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14232 bcc190cf-cafb-0310-a4f2-bffc1f526a37