Revised r9566 slightly to be backwards compatible with versions 1.0 - 1.0.2.

Also documented the change. We prefer to load directory_index.html for static
directory listings, but if you have the (older style) "directory_index"
template and no .html version, we'll load that. This should stop a bunch of
mysterious breakages when people upgrade to Django 1.0.3 or Django 1.1. Also
keeps the docs in sync with released code.

Fixed #9993.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9725 bcc190cf-cafb-0310-a4f2-bffc1f526a37