Adress additional review comments for Extbase introduction

[sypets]

In case we do not add the comments to the Intro as is now and address this later:

issues when using Extbase:

Some other issues when using Extbase: If you want to create an extension that should be extended by other extensions you run into a typical ORM issue, where your domain model cannot be easily extended multiple times (single-table inheritance). Also if you want to hook into the request process early via PSR-15 middlewares, it is possible that Extbase Configuration functionality it cannot easily be loaded up during a PSR-15 middleware. In such a case, it might be a better fit to use a custom "page.typeNum = 12312312" logic for .e.g AJAX Request when Extbase is wanted.

https://github.com/TYPO3-Documentation/TYPO3CMS-Reference-CoreApi/pull/2055#discussion_r948814768

"extender" is a good third-party solution to the issue of extending domain models. https://extensions.typo3.org/extension/extender

https://github.com/TYPO3-Documentation/TYPO3CMS-Reference-CoreApi/pull/2055#discussion_r948996776