Update comment on `Track.nalUnitLengthFieldLength`

This field is also non-zero for h.265 tracks.

PiperOrigin-RevId: 523676455

library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/Track.java

@@ -76,8 +76,8 @@ public final class Track {
   @Nullable public final long[] editListMediaTimes;
 
   /**
-   * For H264 video tracks, the length in bytes of the NALUnitLength field in each sample. 0 for
-   * other track types.
+   * The length in bytes of the NALUnitLength field in each sample. 0 for tracks that don't use
+   * length-delimited NAL units.
    */
   public final int nalUnitLengthFieldLength;