Fixed #20484 again -- added note to field documentation
Merge pull request #1453 from chrismedrela/ticket20484-improvement

Fixed #20484 again -- added note to field documentation
Fixed #20870 -- Documented django.utils.functional.cached_property
Clarify meaning of models.User.is_authenticated()
Form.clean() does not need to return cleaned_data.

If it does, that will be used as the cleaned_data. The default
implementation has been changed to match this change.
Revert change to the default Form.clean()

This means it doesn't break for people who are doing
`cleaned_data = super(FooForm, self).clean()`.
Added more on @cached_property, refs #20870
Merge pull request #1457 from jaimeirurzun/master

Clarified meaning of models.User.is_authenticated()