Simplification of return types. No need to return more mixed than necess...

Follow up on https://github.com/cakephp/cakephp/pull/5084

Ruff ideas on how to make the API simpler.
Also some doc block corrections.

Some tests might not be fine. I can check on that again once we discussed it.
Why aren't the `return null` in `_authRequired()` also changed to `return false` for consistency?