[skip ci]Add comments in chap09_data_coord.md (#12052)

Signed-off-by: Nruiyi.jiang <ruiyi.jiang@zilliz.com>

docs/developer_guides/chap09_data_coord.md

@@ -283,6 +283,7 @@ type InsertMsg struct {
 type DataNode interface {
 	Component
 
+  /// WatchDmChannels watches the channels about data manipulation.
 	WatchDmChannels(ctx context.Context, req *datapb.WatchDmChannelsRequest) (*commonpb.Status, error)
 	// FlushSegments notifies DataNode to flush the segments req provids. The flush tasks are async to this rpc, DataNode will flush the segments in the background.
 	FlushSegments(ctx context.Context, req *datapb.FlushSegmentsRequest) (*commonpb.Status, error)