[1.1.X] Fixed #13103 -- A bit of re-organization to the custom template...

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

Backport of r12868 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12869 bcc190cf-cafb-0310-a4f2-bffc1f526a37