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.
With this way to define handleOnChange (similar for handleOnInput), although it documents `@param`, the generated doc shows handleOnChange/handleOnInput as "members", not "methods", and does not show the signature, nor the doc of the parameter. (http://ibm-js.github.io/delite/docs/api/0.4.0/delite/FormValueWidget.html).