Clarify that `ShuffleOrder` must be consistent in both directions

#minor-release

PiperOrigin-RevId: 453622964

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/ShuffleOrder.java

@@ -24,6 +24,9 @@ import java.util.Random;
  * Shuffled order of indices.
  * Shuffled order of indices.
  *
  *
  * <p>The shuffle order must be immutable to ensure thread safety.
  * <p>The shuffle order must be immutable to ensure thread safety.
+ *
+ * <p>The order must be consistent when traversed both {@linkplain #getNextIndex(int) forwards} and
+ * {@linkplain #getPreviousIndex(int) backwards}.
  */
  */
 @UnstableApi
 @UnstableApi
 public interface ShuffleOrder {
 public interface ShuffleOrder {