[1.2.X] Added crossref targets for a couple of Django exceptions and cleaned...

[1.2.X] Added crossref targets for a couple of Django exceptions and cleaned up related reST markup.

Backport of [15733] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15734 bcc190cf-cafb-0310-a4f2-bffc1f526a37