Update etcdserver/api/membership/cluster.go

Co-authored-by: Benjamin Wang <benjamin.wang@broadcom.com>

etcdserver/api/membership/cluster.go

@@ -814,7 +814,10 @@ func detectDowngrade(cv *semver.Version, nextClusterVersionCompatible bool) erro
 	if !lv.LessThan(*cv) {
 		return nil
 	}
-	// allow 3.4 server to join 3.5 cluster version, since the data is compatible after `etcdutil migrate` the data_dir.
+	// allow 3.4 server to join 3.5 cluster. Note the local data schema will
+	// be automatically migrated to 3.4 if `--next-cluster-version-compatible`
+	// is enabled (true). Users can also execute `etcdutl migrate` to migrate
+	// the data before starting the server.
 	oneMinorVersionDown := &semver.Version{Major: cv.Major, Minor: cv.Minor - 1}
 	if !nextClusterVersionCompatible || !lv.Equal(*oneMinorVersionDown) {
 		return fmt.Errorf("invalid downgrade; (current version: %s is lower than determined cluster version: %s).", version.Version, version.Cluster(cv.String()))