drm/doc: Improve docs for conn_state->best_encoder

It's mandatory and considered core state since ioctls rely on this
working.

Thanks to Laurent for pointing out this gap.

v2: Clarify to "atomic drivers" only.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Sean Paul <sean@poorly.run>
Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190506144629.5976-1-daniel.vetter@ffwll.ch

include/drm/drm_connector.h

@@ -517,6 +517,10 @@ struct drm_connector_state {
 	 * Used by the atomic helpers to select the encoder, through the
 	 * &drm_connector_helper_funcs.atomic_best_encoder or
 	 * &drm_connector_helper_funcs.best_encoder callbacks.
+	 *
+	 * NOTE: Atomic drivers must fill this out (either themselves or through
+	 * helpers), for otherwise the GETCONNECTOR and GETENCODER IOCTLs will
+	 * not return correct data to userspace.
 	 */
 	struct drm_encoder *best_encoder;