Update the doc for backups and restore.

This change updates the existing doc for the new implementation, including details about it.  The changes reflect the state of things once the new backup and restore implementations have landed.
I see what you're saying.  When we cover that case in the new restore, I expect that the --restore subcommand will grow a --filename option to indicate the file to upload.  At that point juju backups --download should be added as the inverse.  It would probably also make sense to add a --download option to --create, though that would just be a convenience and limited to the backup you just created.  I'll reword that part so it doesn't downplay the role of those tools.