vzdump: notes template: document pool variable

Signed-off-by: Michal Fox <me@dualfroz.com>

vzdump.adoc

@@ -445,6 +445,7 @@ Currently supported are:
 * `{{cluster}}` the cluster name, if any
 * `{{guestname}}` the virtual guest's assigned name
 * `{{node}}` the host name of the node the backup is being created
+* `{{pool}}` the resource pool the guest is a member of, if any
 * `{{vmid}}` the numerical VMID of the guest
 
 When specified via API or CLI, it needs to be a single line, where newline and