The thing with UX Markup is that almost any element can be placed in another element, and how they behave directly depends on parent-child relationship. There’s literally thousands of variations you can theoretically end up with, and documenting all of them is virtually impossible, never mind showing that in a meaningful, easy-to-understand way.
There’s way less rules about what you can not do in UX than there is about what you can. Making a definitive list about what’s not allowed perhaps doesn’t make much sense though.
Sure, our docs could use a better structure and other improvements, but do you have a very specific, particular suggestion that we could look at?
If you’re struggling to learn Fuse, get started with Basics (and all subsections there), and then proceed to the Tutorial. Having covered those, you should find yourself comfortable enough to take on more complex things.