Fixed #23376 -- Make documentation about required Storage methods 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 
"Another example would be a backend..."