Fixed #13040 -- Added info on where to import File class from to File...

Fixed #13040 -- Added info on where to import File class from to File reference docs, and improved Sphinx formatting. Thanks to stherrien for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14339 bcc190cf-cafb-0310-a4f2-bffc1f526a37