Clarify that the shutter can prevent flicker

PiperOrigin-RevId: 225374071

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlayerView.java

@@ -186,8 +186,9 @@ import java.util.List;
  *         <li>Type: {@link AspectRatioFrameLayout}
  *       </ul>
  *   <li><b>{@code exo_shutter}</b> - A view that's made visible when video should be hidden. This
- *       view is typically an opaque view that covers the video surface view, thereby obscuring it
- *       when visible.
+ *       view is typically an opaque view that covers the video surface, thereby obscuring it when
+ *       visible. Obscuring the surface in this way also helps to prevent flicker at the start of
+ *       playback when {@code surface_type="surface_view"}
  *       <ul>
  *         <li>Type: {@link View}
  *       </ul>