Added docs for `choice_label_format` option

| Q             | A
| ------------- | ---
| Doc fix?        | no
| New docs?   | yes
| Applies to     | all

Mods can put this on hold; this can be merged if/once PR gets merged: https://github.com/symfony/symfony/pull/12882
For the link, you should use the `doc` role:

```rst
[...], just :doc:`create a Form extension </cookbook/form/create_form_type_extension>` [...]
```