better description for server string

borgmatic/config/schema.yaml

@@ -1628,6 +1628,7 @@ properties:
                 type: string
                 description: |
                     The address of your self-hosted zabbix instance.
+                    Not specifying a server will default to the Zabbix cloud.
                 example: https://zabbix.your-domain.com
             username:
                 type: string