I'm not sure why it complains about this here and not elsewhere, but this
seems to make gi-docgen not warn anymore.

If the pointer is still within hovered_at rectangle, keep the assistant
displayed.

Related: GNOME/gnome-builder#2241

- Replace scope `async` by `notified`, as the callback can be called
  multiple times
- Properly document closure and destroy notification
- Language bindings *need* the destroy notification to do their memory
  management magic, so remap `gtk_source_scheduler_add_full()` to the
  actually useless `gtk_source_scheduler_add()`