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
The following code block isn't really PHP (`text` seems to match best). At the end, this part could basically be:

```rst
In the example above, the result labels will finally look like this:

.. code-block:: text

    my_form.choices.fruits.apple
    my_form.choices.fruits.banana
    my_form.choices.fruits.cherry
    ...
    my_form.choices.fruits.cherry

```