permission plugin: Improve description of the target option

https://fedorahosted.org/freeipa/ticket/4521

Reviewed-By: Martin Kosek <mkosek@redhat.com>

ipalib/plugins/permission.py

@@ -296,7 +296,9 @@ class permission(baseldap.LDAPObject):
         DNParam(
             'ipapermtarget?',
             cli_name='target',
-            label=_('ACI target DN'),
+            label=_('Target DN'),
+            doc=_('Optional DN to apply the permission to '
+                  '(must be in the subtree, but may not yet exist)'),
         ),
 
         Str('memberof*',