openstack volume backup list --volume X uses /v3/.../backups/detail?volume_id=... whereas gophercloud uses /v3/.../backups?volume_id=.... detail adds the backup status which can be useful for filtering out failed backups (significantly reduces number of requests if there are a lot).

I'd be glad to make a PR adding this if someone can give me an overview of the form it should take. New ListDetail function and listDetailUrl which takes the same query struct?