Update changelog for storage format changes.

CHANGELOG-3.5.md

@@ -73,6 +73,11 @@ See [code changes](https://github.com/etcd-io/etcd/compare/v3.4.0...v3.5.0) and
 - [Embed Etcd does not override global/grpc logger](https://github.com/etcd-io/etcd/pull/12861) be default any longer. If desired, please call `embed.Config::SetupGlobalLoggers()` explicitly. 
 - Client errors of `context cancelled` or `context deadline exceeded` are exposed as `codes.Canceled` and `codes.DeadlineExceeded`, instead of `codes.Unknown`.
 
+### Storage format changes
+- [WAL log's snapshots persists raftpb.ConfState](https://github.com/etcd-io/etcd/pull/12735)
+- [Backend persists raftpb.ConfState](https://github.com/etcd-io/etcd/pull/12962) in the `meta` bucket `confState` key.
+- Backend persists downgrade in the `cluster` bucket
+
 ### Security
 
 - Add [`TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256` and `TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256` to `etcd --cipher-suites`](https://github.com/etcd-io/etcd/pull/11864).