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

include/uapi/drm/drm.h

@@ -678,7 +678,9 @@ struct drm_get_cap {
 /**
  * DRM_CLIENT_CAP_ATOMIC
  *
- * If set to 1, the DRM core will expose atomic properties to userspace
+ * If set to 1, the DRM core will expose atomic properties to userspace. This
+ * implicitly enables &DRM_CLIENT_CAP_UNIVERSAL_PLANES and
+ * &DRM_CLIENT_CAP_ASPECT_RATIO.
  */
 #define DRM_CLIENT_CAP_ATOMIC	3