The (closure) annotation should go on the `callback` parameter, e.g.

```
 * @callback: (scope async) (closure user_data): a callback to call when the operation is complete
```

Alternatively, when the user_data parameter is annotated, the annotation
requires no arguments.

However, heuristic number 378 picks up parameters named `user_data`
automatically as the callback's user data if there is a callback.