more doc

modules/elasticsearch/src/main/java/org/elasticsearch/index/cache/bloom/BloomCache.java

@@ -30,7 +30,8 @@ import org.elasticsearch.index.IndexComponent;
 public interface BloomCache extends IndexComponent, CloseableComponent {
 
     /**
-     * *Async* loads a bloom filter for the field name.
+     * *Async* loads a bloom filter for the field name. Note, this one only supports
+     * for fields that have a single term per doc.
      */
     BloomFilter filter(IndexReader reader, String fieldName, boolean asyncLoad);
 

modules/elasticsearch/src/main/java/org/elasticsearch/index/shard/service/InternalIndexShard.java

@@ -579,7 +579,8 @@ public class InternalIndexShard extends AbstractIndexShardComponent implements I
             }
         }
         // since we can do async merging, it will not be called explicitly when indexing (adding / deleting docs), and only when flushing
-        // so, make sure we periodically call it
+        // so, make sure we periodically call it, this need to be a small enough value so mergine will actually
+        // happen and reduce the number of segments
         TimeValue optimizeInterval = indexSettings.getAsTime("index.merge.async_interval", TimeValue.timeValueSeconds(1));
         if (optimizeInterval.millis() > 0) {
             optimizeScheduleFuture = threadPool.scheduleWithFixedDelay(new EngineOptimizer(), optimizeInterval);