drm: better document how to send out the crtc disable event

The kernel doc explained what needs to happen, but not how to most
easily accomplish that using the functions. Fix that.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-14-daniel.vetter@ffwll.ch

include/drm/drm_crtc.h

@@ -214,7 +214,9 @@ struct drm_crtc_state {
 	 *    atomic commit. In that case the event can be send out any time
 	 *    after the hardware has stopped scanning out the current
 	 *    framebuffers. It should contain the timestamp and counter for the
-	 *    last vblank before the display pipeline was shut off.
+	 *    last vblank before the display pipeline was shut off. The simplest
+	 *    way to achieve that is calling drm_crtc_send_vblank_event()
+	 *    somewhen after drm_crtc_vblank_off() has been called.
 	 *
 	 *  - For a CRTC which is enabled at the end of the commit (even when it
 	 *    undergoes an full modeset) the vblank timestamp and counter must