doc: made functions taking wl_event_queue arg belong to wl_event_queue

The fact that these functions take both a display and queue argument is
I think historical, and they really are methods on the queue.

Also added some docs for wl_display_prepare_read_queue.

Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>