[WIP] field dependent empty_data option description

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4007

- [ ] checkbox
- [x] choice
- [x] collection
- [x] country
- [x] currency
- [x] email
- [x] entity
- [x] file
- [ ] form
- [x] integer
- [x] language
- [x] locale
- [x] money
- [x] number
- [x] password
- [x] percent
- [ ] radio
- [x] search
- [x] text
- [x] textarea
- [x] timezone
- [x] url

This is not yet complete. But I'm not sure if I like the way it looks now with the default value below the ``note`` directive.

This is accidentally opened against the master branch. Though it should get merged into the ``2.3`` branch.
this example is now gone and with that, the usecase of the option is no longer made clear.