add note on not using totalSize in merge

src/main/java/org/apache/lucene/index/TrackingConcurrentMergeScheduler.java

@@ -78,6 +78,7 @@ public class TrackingConcurrentMergeScheduler extends ConcurrentMergeScheduler {
     @Override
     protected void doMerge(MergePolicy.OneMerge merge) throws IOException {
         int totalNumDocs = merge.totalNumDocs();
+        // don't used #totalBytesSize() since need to be executed under IW lock, might be fixed in future Lucene version
         long totalSizeInBytes = merge.estimatedMergeBytes;
         long time = System.currentTimeMillis();
         currentMerges.inc();

src/main/java/org/apache/lucene/index/TrackingSerialMergeScheduler.java

@@ -88,6 +88,7 @@ public class TrackingSerialMergeScheduler extends MergeScheduler {
             writer.mergeInit(merge);
 
             int totalNumDocs = merge.totalNumDocs();
+            // don't used #totalBytesSize() since need to be executed under IW lock, might be fixed in future Lucene version
             long totalSizeInBytes = merge.estimatedMergeBytes;
             long time = System.currentTimeMillis();
             currentMerges.inc();