[skip e2e] Add comment for rootCood.Core (#14665)

Signed-off-by: shaoyue.chen <shaoyue.chen@zilliz.com>

internal/rootcoord/root_coord.go

@@ -937,6 +937,7 @@ func (c *Core) Register() error {
 	return nil
 }
 
+// SetEtcdClient sets the etcdCli of Core
 func (c *Core) SetEtcdClient(etcdClient *clientv3.Client) {
 	c.etcdCli = etcdClient
 }