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.
(Minor) Alternate variant:
... where the user can interactively change the `value` property (by using the mouse, keyboard, touch...).
(open enumeration because of speach interaction etc.)