client: Improve wl_display_roundtrip(_queue)() documentation

Change the wording a bit to describe how it is done (which explains to
the name of the function) as well as a note about that we actually will
dispatch events that are received.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>