Add wernings about bad practice with config split

Problem/Motivation

Some people are confused what config split does and may think that they need to split core.extensions or config split itself or may not understand that splitting splits is a tricky thing to do.

Steps to reproduce

see #3540534: Complete module splits not enabling modules

Proposed resolution

$
Add documentation page explaining in detail why certain configuration is not recommended.
Add warnings when saving a split (like if the split folder doesn't exist) if some non standard setup is detected.
Add warnings to the status page. (link to documentation page)
Add an option to settings.php that lets developers suppress the warnings.

Remaining tasks

create documentation page
create patch.

User interface changes

new warnings may pop up.

API changes

none: we still let people do what they want to do, even if they probably shouldn't.

Data model changes

none