Document error case for generateAudioSessionIdV21

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=220132865

library/core/src/main/java/com/google/android/exoplayer2/C.java

@@ -978,7 +978,10 @@ public final class C {
   }
 
   /**
-   * Returns a newly generated {@link android.media.AudioTrack} session identifier.
+   * Returns a newly generated audio session identifier, or {@link AudioManager#ERROR} if an error
+   * occurred in which case audio playback may fail.
+   *
+   * @see AudioManager#generateAudioSessionId()
    */
   @TargetApi(21)
   public static int generateAudioSessionIdV21(Context context) {