Update release notes to the latest bugfix

Addressed by #931 .
From hipchat:

```
(15時55分26秒) Shunichi Shinohara: list buckets -> get bucket and confirm delete-marker -> deleting MPs ...(cont) -> user create bucket and init MP (at 1.4.x nodes) -> (cont) delete just inited MP
(15時55分35秒) Shunichi Shinohara: broken, true
```