Fixed #14537 -- Added documentation on where validators can be imported from...

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.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14331 bcc190cf-cafb-0310-a4f2-bffc1f526a37