[skip ci] Add comment for addCollectionFlowGraph (#8606)

Signed-off-by: Nbigsheeper <yihao.dai@zilliz.com>

internal/querynode/data_sync_service.go

@@ -44,6 +44,7 @@ type dataSyncService struct {
 }
 
 // collection flow graph
+// addCollectionFlowGraph add a collection flowGraph to collectionFlowGraphs
 func (dsService *dataSyncService) addCollectionFlowGraph(collectionID UniqueID, vChannels []string) {
 	dsService.mu.Lock()
 	defer dsService.mu.Unlock()