protocol: mention that buffers with alpha are assumed premultiplied
This turns out to be the default assumption by compositors, OpenGL, and the DRM framework. Let's say it explicitly to avoid misinterpretation.
See https://lists.freedesktop.org/archives/wayland-devel/2021-September/041990.html.
Signed-off-by: Matt Hoosier matt.hoosier@garmin.com