[Form] Added information about float choice lists

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Added an information about the possibility to have a choice list with float values.

Relates to symfony/symfony#11849
using the ``::`` shortcut will add a ``:`` at the end of the paragraph. This is wrong here, as your sentence already ends with a dot.
There is 2 solutions here, depending on whether you want the ``:`` in the output:

- removing the dot
- using the ``.. code-block:: php`` directive rather than the shortcut