[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
I don't think the doctrine update should be mentionned. It is not even the main issue. There is worse than the noop update: in case of edition, the wrong value will be initially selected when using the wrong type, because Symfony uses strict comparison to choose the value