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.

Start by locating the release tool's CHANGELOG entry generation step and reproduce a release whose entry contains only the heading and special thanks. Confirm the warning tells the Caretaker to update the CHANGELOG, including the patch-release PR generated against the master branch; done means both cases provide that indication.

Written by the indexing model from the issue text.