add comnments

Signed-off-by: ystaticy <y_static_y@sina.com>

domain/infosync/info.go

@@ -95,7 +95,12 @@ var ErrPrometheusAddrIsNotSet = dbterror.ClassDomain.NewStd(errno.ErrPrometheusA
 
 // InfoSyncer stores server info to etcd when the tidb-server starts and delete when tidb-server shuts down.
 type InfoSyncer struct {
-	etcdCli           *clientv3.Client
+	// `etcdClient` must be used when keyspace is not set, or when the logic to each etcd path needs to be separated by keyspace.
+	etcdCli *clientv3.Client
+	// `unprefixedEtcdCli` will never set the etcd namespace prefix by keyspace.
+	// It is only used in storeMinStartTS and RemoveMinStartTS now.
+	// It must be used when the etcd path isn't needed to separate by keyspace.
+	// See keyspace RFC: https://github.com/pingcap/tidb/pull/39685
 	unprefixedEtcdCli *clientv3.Client
 	info              *ServerInfo
 	serverInfoPath    string