[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 think we can rephrase this a bit (and also link to the ``choice_list`` option):

```rst
.. tip::

    When the values to choose from are not integers or strings (but e.g. floats
    or booleans), you should use the `choice_list`_ option instead. With this
    option you are able to keep the original data format which is important
    to ensure that the user input is validated properly.
```

I'm not sure if the part about database updates is that important. What do you think about this?