[skip ci]Add notes to datanode service (#13209)

Signed-off-by: jingkl <jingjing.jia@zilliz.com>

internal/distributed/datanode/service.go

@@ -282,6 +282,7 @@ func (s *Server) start() error {
 	return nil
 }
 
+// GetComponentStates gets the component states of Datanode
 func (s *Server) GetComponentStates(ctx context.Context, req *internalpb.GetComponentStatesRequest) (*internalpb.ComponentStates, error) {
 	return s.datanode.GetComponentStates(ctx)
 }