CHANGELOG-4.0: deprecate "etcdctl backup" command

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

CHANGELOG-4.0.md

@@ -15,8 +15,9 @@ See [code changes](https://github.com/coreos/etcd/compare/v3.3.0...v4.0.0) and [
   - Now, `{"health":true}`.
   - Breaks [Kubernetes `kubectl get componentstatuses` command](https://github.com/kubernetes/kubernetes/issues/58240).
 - Deprecate [`etcd --proxy*`](TODO) flags; **no more v2 proxy**.
-- Deprecate [v2 storage backend](https://github.com/coreos/etcd/issues/9232).
+- Deprecate [v2 storage backend](https://github.com/coreos/etcd/issues/9232); **no more v2 store**.
   - v2 API is still supported via [v2 emulation](TODO).
+- Deprecate [`etcdctl backup`](TODO) command.
 - `clientv3.Client.KeepAlive(ctx context.Context, id LeaseID) (<-chan *LeaseKeepAliveResponse, error)` is now [`clientv4.Client.KeepAlive(ctx context.Context, id LeaseID) <-chan *LeaseKeepAliveResponse`](TODO).
   - Similar to `Watch`, [`KeepAlive` does not return errors](https://github.com/coreos/etcd/issues/7488).
   - If there's an unknown server error, kill all open channels and create a new stream on the next `KeepAlive` call.