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

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Signed-off-by: Matt Hoosier <matt.hoosier@garmin.com>