[1.2.X] Fixed #14537 -- Added documentation on where validators can be...

[1.2.X] Fixed #14537 -- Added documentation on where validators can be imported from to Validators Reference docs, and improved Sphinx formatting and metadata. Thanks to rfugger for the report.

Backport of [14331] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14332 bcc190cf-cafb-0310-a4f2-bffc1f526a37