Release tool: provide an indication when CHANGELOG entry has no content #308

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:

<a name="13.0.2"></a>
# 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

<!-- CHANGELOG SPLIT MARKER -->

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.