add comment about de-serialing bySegment

server/src/main/java/io/druid/client/DirectDruidClient.java

@@ -268,6 +268,8 @@ public class DirectDruidClient<T> implements QueryRunner<T>
         }
     );
 
+    // bySegment queries are de-serialized after caching results in order to
+    // avoid the cost of de-serializing and then re-serializing again when adding to cache
     if (!isBySegment) {
       retVal = Sequences.map(
           retVal,