virsh: describe attach-interface parameter target

This patch is based on a improvement suggested by Kazuhiro Kikuchi
of Fujitsu, it gives a description of the target parameter for that
command

* tools/virsh.pod: add description for target parameter of
  attach-interface

tools/virsh.pod

@@ -1239,6 +1239,10 @@ the default one.
 I<model> allows to specify the model type.
 I<persistent> indicates the changes will affect the next boot of the domain.
 
+B<Note>: the optional target value is the name of a device to be created
+as the back-end on the node. If not provided a device named "vnetN" or "vifN"
+will be created automatically.
+
 =item B<detach-device> I<domain-id> I<FILE>
 
 Detach a device from the domain, takes the same kind of XML descriptions