[WIP][Reference][Form Types] Update "radio" form type

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3410

`radio`'s parent is `checkbox`, not `form`.

I need some help regarding the task *value option should be removed from this type*:
Should it really be removed? It's because I'm able to set it and it is used in the HTML output. I'm retrieving a boolean value only when I access the submitted data via the form framework but I have also access to the raw, custom value via e.g. `$request->request->get('my_checkbox')`. So maybe it should be moved to the `inherited options` instead for the `radio` field? If yes, the wording would need an update for the `value` option and if no, why keep it for the `checkbox` field?