add comment about confusing constant value

core/src/main/java/org/elasticsearch/index/store/IndexStoreConfig.java

@@ -34,8 +34,10 @@ import org.elasticsearch.node.settings.NodeSettingsService;
  */
 public class IndexStoreConfig implements NodeSettingsService.Listener {
 
-
+    /** "Effectively" infinite (20 GB/sec) default value, because store throttling is disabled by default since
+     *  we use Lucene's auto-IO throttling instead. */ 
     private static final ByteSizeValue DEFAULT_THROTTLE = new ByteSizeValue(10240, ByteSizeUnit.MB);
+
     /**
      * Configures the node / cluster level throttle type. See {@link StoreRateLimiting.Type}.
      */