Release tool: provide an indication when CHANGELOG entry has no content

One of the steps of the release process is CHANGELOG entry generation. Sometimes the entry might have no content (for example when only docs/refactoring changes were merged since the previous release) and the entry looks like this:

```

# 13.0.2 (2021-11-17)
## Special Thanks
Andrew Kushnir, Armen Vardanyan, Dylan Hunn, Joey Perrott, Martin von Gagern, Paul Gschwendtner, Pete Bacon Darwin, Ramesh Thiruchelvam, dario-piotrowicz and fusho-takahashi

```

Release instructions used to suggest that Caretakers should add the following phrase in this case:

```
This release contains various API docs improvements.
```

The proposal is to warn a Caretaker that actions need to be taken to update the CHANGELOG.

P.S.: issue #263 also results in adding the same update to the PR generated against the master branch during the patch release.

まず、release tool の CHANGELOG エントリ生成ステップを特定し、そのエントリが見出しと特別な謝辞だけを含む release を再現します。警告が Caretaker に CHANGELOG を更新するよう伝えることを確認します。これには master branch に対して生成された patch-release PR も含まれます。両方のケースでその指示が示されれば完了です。

索引モデルが issue の本文から書いたものです。