`juju help create-backup` refers to `keep-copy`

[John A Meinel]

I believe in 2.9 we deprecated supporting --keep-copy and storing the result of the backup in the database.
However, help still refers to it without actually explaining it:
```
$ juju create-backup --help
Usage: juju create-backup [options] [<notes>]
...
--no-download (= false)
    Do not download the archive, implies keep-copy
...
```

I believe we want to deprecate --no-download in the same way that we deprecated --keep-copy.

If we have a reason we cannot, we should change the help text to explain what we mean, rather than referencing something that doesn't exist.