Trivial: Add missing doc for parameter in wait_for_delete().

The doc of parameter manager is missing.

Change-Id: I4e99c06ab713532d73615670ada0a61462285d76

Author: tang-chen

openstackclient/common/utils.py

@@ -335,6 +335,7 @@ def wait_for_delete(manager,
                     callback=None):
     """Wait for resource deletion
 
+    :param manager: the manager from which we can get the resource
     :param res_id: the resource id to watch
     :param status_field: the status attribute in the returned resource object,
         this is used to check for error states while the resource is being