Clarify some threading requirements.

PiperOrigin-RevId: 512079471

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/AssetLoader.java

@@ -90,8 +90,8 @@ public interface AssetLoader {
      * <p>Must be called once per {@linkplain #onTrackCount(int) declared} track.
      *
      * <p>Must be called from the thread that will be used to call the returned {@link
-     * SampleConsumer}'s methods. This thread is generally different from the one used to access the
+     * SampleConsumer}'s methods. This thread must be the same for all the tracks added, and is
-     * {@link AssetLoader} methods.
+     * generally different from the one used to access the {@link AssetLoader} methods.
      *
      * @param format The {@link Format} of the input media (prior to video slow motion flattening or
      *     to decoding).

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/CompositeAssetLoader.java

@@ -146,7 +146,7 @@ import java.util.concurrent.atomic.AtomicInteger;
    *
    * <p>There can't be more than one {@link OnMediaItemChangedListener} for the same track type.
    *
-   * <p>Can be called from any thread.
+   * <p>Must always be called from the same thread. This thread can be any thread.
    *
    * @param onMediaItemChangedListener The {@link OnMediaItemChangedListener}.
    * @param trackType The {@link C.TrackType} for which to listen to {@link MediaItem} change