backups create help text is wrong about downloading

[Aaron Bentley]

Download is now the default, as of https://github.com/juju/juju/pull/1037 but the help text is from a previous version.

Not only does it refer to --download, it implies that if it (or --filename) is not used, the file will not be downloaded.

Here is the current text:
"create" requests that juju create a backup of its state and print the
backup's unique ID. You may provide a note to associate with the backup.

The backup archive and associated metadata are stored remotely by juju.

The --download option may be used without the --filename option. In
that case, the backup archive will be stored in the current working
directory with a name matching juju-backup-<date>-<time>.tar.gz.

WARNING: Remotely stored backups will be lost when the environment is
destroyed. Furthermore, the remotely backup is not guaranteed to be
available.

Therefore, you should use the --download or --filename options, or use
"juju backups download", to get a local copy of the backup archive.
This local copy can then be used to restore an environment even if that
environment was already destroyed or is otherwise unavailable.

[Anastasia (anastasia-macmood)]

PR against develop (2.3+): https://github.com/juju/juju/pull/7986