I’m not really sure what is the goal of your post… documentation can never be perfect and fit different level user experience and expectations. I feel the docs related to setup_warning are “good enough”. Nextcloud supports huge number of platforms and installation methods so it is impossible to provide a docs which perfectly fit each system.
At the moment the is a bug related to the point 1: Mimetype Warning after upgrading to 28.0.9 or 29.0.5. Second on is more than clear and have been often discussed here - use search! the 3rd one is little more complex but here again many different supported platforms make it hard to create good docs…
The project is open source and everybody is welcome to help. If you feel current docs are not good - you are welcome to provide better variant.