protocol: wl_shm.format events are actually sent after bind

Protocol documentation mistakenly specified that wl_shm.format
events are sent at connection setup time, but that's impossible
(clients do not yet know ID of wl_shm object at the time) and
contradicts implementation in libwayland.

Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>