add javadoc

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1303015 13f79535-47bb-0310-9956-ffa450edef68

lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java

@@ -258,6 +258,8 @@ public class TieredMergePolicy extends MergePolicy {
 
   private final Comparator<SegmentInfo> segmentByteSizeDescending = new SegmentByteSizeDescending();
 
+  /** Holds score and explanation for a single candidate
+   *  merge. */
   protected static abstract class MergeScore {
     abstract double getScore();
     abstract String getExplanation();