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.
It would be welcome to have more information about FormValueWidget.readOnly. Beside the mentioning here, there is a sentence in the doc of the property, but, in particular, it doesn't say that widget's `valueNode` needs to support a `readOnly` property/attribute. That said, I must admit I'm not sure I understand the logic of the code related with this property in FormValueWidget.refreshRendering() (https://github.com/wkeese/delite/blob/formwidget-doc/FormValueWidget.js#L78). But precisely because it's tricky a doc explanation would help (assuming the code is correct and doesn't need changes before documenting it).