[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
Well I am unsure about this. However as a side effect you have those updates which might cause updated timestamps, called listeners just based on an altered value. I have added it again to raise awareness, however I am fine with removing it. You guys can decide.