rewrite api doc for FormWidget and FormValueWidget, fixes #18363

cc @AdrianVasiliu 

Hopefully this addresses all the points you brought up.  You can add some inline comments if you see places for further clarification.
I agree, it would be better to be consistent accross all the code (and not only in delite), and this concerns not only class names but also class members (where backticks might be even more useful). Whether it's better to do it everywhere at the same time or incrementally (given that we're already inconsistent), I don't have a preference. Anyway, not a big deal, though.