drm/doc: the KMS properties section is for user-space devs

State that the "KMS Properties" section is mainly for user-space
developers.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201217113220.102271-2-contact@emersion.fr