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.
OK... well that may be a good idea, but most of our other code doesn't use backticks for class names.   For example, delite/Container, delite/StoreMap, deliteful/Button.  Perhaps because it's obvious they are class names?   Anyway, I guess if we do add backticks for class names we should do it across all the code at the same time.