[1.2.X] Fixed #13040 -- Added info on where to import File class from to File...

[1.2.X] 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.

Backport of [14339] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14340 bcc190cf-cafb-0310-a4f2-bffc1f526a37