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

Documentation/gpu/drm-kms.rst

@@ -460,6 +460,9 @@ KMS Locking
 KMS Properties
 ==============
 
+This section of the documentation is primarily aimed at user-space developers.
+For the driver APIs, see the other sections.
+
 Property Types and Blob Property Support
 ----------------------------------------