Fixed #13103 -- A bit of re-organization to the custom template `ModelAdmin`...

Fixed #13103 -- A bit of re-organization to the custom template `ModelAdmin` options documentation section to remove some duplication and fix some confusing wording.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12868 bcc190cf-cafb-0310-a4f2-bffc1f526a37