add comments

weed/server/filer_grpc_server_remote.go

@@ -60,6 +60,7 @@ func (fs *FilerServer) DownloadToLocal(ctx context.Context, req *filer_pb.Downlo
 	}
 
 	// detect storage option
+	// replication level is set to "000" to ensure only need to ask one volume server to fetch the data.
 	so, err := fs.detectStorageOption(req.Directory, "", "000", 0, "", "", "")
 	if err != nil {
 		return resp, err