Fix documentation confusion in MediaChunkIterator subclasses

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217505571

library/core/src/main/java/com/google/android/exoplayer2/source/chunk/BaseMediaChunkIterator.java

@@ -31,7 +31,7 @@ public abstract class BaseMediaChunkIterator implements MediaChunkIterator {
   /**
    * Creates base iterator.
    *
-   * @param fromIndex The index at which the iterator will start.
+   * @param fromIndex The first available index.
    * @param toIndex The last available index.
    */
   public BaseMediaChunkIterator(long fromIndex, long toIndex) {

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DefaultDashChunkSource.java

@@ -578,12 +578,14 @@ public class DefaultDashChunkSource implements DashChunkSource {
      * Creates iterator.
      *
      * @param representation The {@link RepresentationHolder} to wrap.
-     * @param segmentNum The number of the segment this iterator will be pointing to initially.
+     * @param firstAvailableSegmentNum The number of the first available segment.
      * @param lastAvailableSegmentNum The number of the last available segment.
      */
     public RepresentationSegmentIterator(
-        RepresentationHolder representation, long segmentNum, long lastAvailableSegmentNum) {
-      super(/* fromIndex= */ segmentNum, /* toIndex= */ lastAvailableSegmentNum);
+        RepresentationHolder representation,
+        long firstAvailableSegmentNum,
+        long lastAvailableSegmentNum) {
+      super(/* fromIndex= */ firstAvailableSegmentNum, /* toIndex= */ lastAvailableSegmentNum);
       this.representationHolder = representation;
     }
 

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsChunkSource.java

@@ -631,7 +631,7 @@ import java.util.List;
      * @param playlist The {@link HlsMediaPlaylist} to wrap.
      * @param startOfPlaylistInPeriodUs The start time of the playlist in the period, in
      *     microseconds.
-     * @param chunkIndex The chunk index in the playlist at which the iterator will start.
+     * @param chunkIndex The index of the first available chunk in the playlist.
      */
     public HlsMediaPlaylistSegmentIterator(
         HlsMediaPlaylist playlist, long startOfPlaylistInPeriodUs, int chunkIndex) {

library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/DefaultSsChunkSource.java

@@ -318,7 +318,7 @@ public class DefaultSsChunkSource implements SsChunkSource {
      *
      * @param streamElement The {@link StreamElement} to wrap.
      * @param trackIndex The track index in the stream element.
-     * @param chunkIndex The chunk index at which the iterator will start.
+     * @param chunkIndex The index of the first available chunk.
      */
     public StreamElementIterator(StreamElement streamElement, int trackIndex, int chunkIndex) {
       super(/* fromIndex= */ chunkIndex, /* toIndex= */ streamElement.chunkCount - 1);