Add a sentence to TrackSelector javadoc calling out #invalidate()

PiperOrigin-RevId: 325767964

library/core/src/main/java/com/google/android/exoplayer2/trackselection/TrackSelector.java

@@ -61,6 +61,8 @@ import com.google.android.exoplayer2.util.Assertions;
  *       prefer audio tracks in a particular language. This will trigger the player to make new
  *       prefer audio tracks in a particular language. This will trigger the player to make new
  *       track selections. Note that the player will have to re-buffer in the case that the new
  *       track selections. Note that the player will have to re-buffer in the case that the new
  *       track selection for the currently playing period differs from the one that was invalidated.
  *       track selection for the currently playing period differs from the one that was invalidated.
+ *       Implementing subclasses can trigger invalidation by calling {@link #invalidate()}, which
+ *       will call {@link InvalidationListener#onTrackSelectionsInvalidated()}.
  * </ul>
  * </ul>
  *
  *
  * <h3>Renderer configuration</h3>
  * <h3>Renderer configuration</h3>