drm/doc: atomic implicitly enables other caps

Document that enabling atomic also enables universal planes and aspect
ratio modes.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201217113220.102271-7-contact@emersion.fr