I don't think it is normal to document literally every setting, feature and button of an application if they are somewhat self-documenting - the setting in question already has a description in the setting wording. It was included in the change notes (granted I think the wording isn't as clear as it could be but it wasn't some hidden setting with a complicated way of changing the value).

I don't quite understand what is meant by this? What exactly is this reason and how does an appearance setting relate to the mobile editor?
As for editing on mobile, the new beta editor is certainly better than the old one, the main issue for me is just inputting markdown via a stock mobile keyboard is a bit of a faff (mostly making task lists).