rewrite api doc for FormWidget and FormValueWidget, fixes #329

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 added explicit documentation for `focusNode` and `valueNode` in e8944f96e2f80732193927b4fc812f65cc20c26e, and I updated the wording there.