protocol: add better description of wl_pointer.release

Add note about what all wl_pointer.release does. Mainly that
it destroys the proxy object, so programmer must not call
wl_pointer_destroy() on the pointer any further.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>