returntypewillchange.xml Add warning about future strict return type enforcement

[mmalferov]

Should we not explicitly mention that internal methods will strictly enforce return types in future PHP versions?

Currently, the documentation might give developers a false sense of security, making it look like #[ReturnTypeWillChange] is a permanent fix. In reality, it only works during the tentative phase. Once PHP 9.0 drops, any remaining signature mismatches in child classes will cause a fatal error, as far as I understand.

P. S. The checks did not pass in the previous PR. And something went wrong with rebase. So I created a new Pull Request, and closed the old one:

#5608

But even now the check doesn 't work 🤷