consistent.
The following methods **should** be implemented, but are not required:
* Storage.delete()
* Storage.exists()
* Storage.listdir()
* Storage.size()
* Storage.url()
Updated documentation to reflect this fact and give a couple of examples
where some methods may not be implemented. Add a warning that not
implementing some methods will result in a partial (possibly broken)
interface.
Ticket: https://code.djangoproject.com/ticket/23376
332706eaa0