Drop the old notes about performance benefits, they are already
explained with preferred_changed. Also don't emphasize getting image
descriptions from outputs, that should not be a usual option. Only
display profiling software should use output-specific image
descriptions.

Maybe a good idea to mention that clients must understand the image
description and not just get_preferred, blindly slap it on the
wl_surface, and render without any adjustments.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>