get_backup_volumes: clarifcation and indentation improvements to comment

get_backup_volumes: clarifcation and indentation improvements to comment

Aaron clarified that one documented element actually slipped through
and won't be returned.

Also firx the mixed tab/space indentation, for such comments we can
be pretty liberal and I just went with the two spaces per level, as
some part already had that.

Clarify that both array and hash are refs.

[0]: https://pve.proxmox.com/pipermail/pve-devel/2020-March/042070.html

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>