fix: Update the limitations page to include volume backups

The volume backup service is not yet configured in any of the
Cleura Cloud regions, so we point out that any volume backups
created using the OpenStack CLI cannot be deleted.

docs/reference/limitations/openstack.md

@@ -20,6 +20,13 @@ It also means that you cannot use the following `openstack` CLI commands; they a
   > You can use `openstack project list`, but this will only list the project(s) that your user account has access to.
 * `openstack domain <subcommand>`
 
+## Cinder
+
+### Volume backup service
+
+The volume backup service is not yet configured in any of the {{brand}} regions.
+As a result, volume backups created using the OpenStack CLI *cannot* be deleted.
+
 ## Nova
 
 ### Nested virtualization