Small docs clarification

docs/api.md

@@ -503,6 +503,7 @@ Identical to the `docker pull` command.
 * stream (bool): Stream the output as a generator
 * insecure_registry (bool): Use an insecure registry
 * auth_config (dict): Override the credentials that Client.login has set for this request
+  `auth_config` should contain the `username` and `password` keys to be valid.
 
 **Returns** (generator or str): The output