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
We should add a comma after "In the given example". And "translateable" should become "translatable".