drm/doc: clarify how to acquire required vblank event reference

As a driver write it is not entirely obvious that a reference to
the event e mentioned in the doc can be obtained via
drm_crtc_vblank_get(). Clarify how to obtain the reference.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180807201143.7446-1-stefan@agner.ch

drivers/gpu/drm/drm_vblank.c

@@ -873,8 +873,8 @@ static void send_vblank_event(struct drm_device *dev,
  * handler by calling drm_crtc_send_vblank_event() and make sure that there's no
  * possible race with the hardware committing the atomic update.
  *
- * Caller must hold a vblank reference for the event @e, which will be dropped
- * when the next vblank arrives.
+ * Caller must hold a vblank reference for the event @e acquired by a
+ * drm_crtc_vblank_get(), which will be dropped when the next vblank arrives.
  */
 void drm_crtc_arm_vblank_event(struct drm_crtc *crtc,
 			       struct drm_pending_vblank_event *e)