Explain carefully why we need two roundtrips, not just one, not just
dispatch and roundtrip, but two roundtrips after creating the
wl_registry object.
v2: Explain what initial events are, and that this is a general
technique.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Marek Chalupa <mchqwerty@gmail.com>