Add guidance about using DOMTokenList

[josepharhar]

What is the issue with the DOM Standard?

It was suggested here that we should add guidance about when to use DOMTokenList (as opposed to setlike?): whatwg/html#8467 (comment)

I suppose there was some talk about setlike supporting "non-boolean" states which is why setlike was chosen instead of DOMTokenList for CSS custom state, but I'm not entirely sure if that's what we should be saying with regards to guidance.

@annevk