[Form] `EnumType`: `group_by` is missing

[ThomasLandauer]

group_by is missing on https://symfony.com/doc/current/reference/forms/types/enum.html

I'm not sure if the text from https://symfony.com/doc/current/reference/forms/types/choice.html#group-by can be reused, cause I don't know how this multi-dimensional array thing would work:

You can group the elements of a <select> into <optgroup> by passing a multi-dimensional array to choices

[alexandre-daubois]

I'm taking a look at this