virsh: man: Mention that volumes need to be in storage pool for undefine

https://bugzilla.redhat.com/show_bug.cgi?id=1044445

When undefining a VM with storage the man page doesn't explicitly
mention that the volumes need to be a part of the storage pool otherwise
it won't work.

tools/virsh.pod

@@ -1794,7 +1794,8 @@ on inactive domains. Volume deletion is only attempted after the domain is
 undefined; if not all of the requested volumes could be deleted, the
 error message indicates what still remains behind. If a volume path is not
 found in the domain definition, it's treated as if the volume was successfully
-deleted.
+deleted. Only volumes managed by libvirt in storage pools can be removed this
+way.
 (See B<domblklist> for list of target names associated to a domain).
 Example: --storage vda,/path/to/storage.img