[Reference] field dependent empty_data option description

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4007

- [x] choice
- [x] collection
- [x] country
- [x] currency
- [x] email
- [x] entity
- [x] file
- [x] form
- [x] integer
- [x] language
- [x] locale
- [x] money
- [x] number
- [x] password
- [x] percent
- [x] 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.

Replacement of #4033.
I think we might be able to put a comment above this, to explain its purpose (I'm not sure if the multi-line comment works like this):

```rst
.. This file should only be included with start-after or end-before that's set to
   this DEFAULT_PLACEHOLDER value. Its purpose is to let us include only part of this file.
```