[Form] Minor reword in the Choice type

Author: javiereguiluz

reference/forms/types/choice.rst

@@ -52,8 +52,9 @@ To use this field, you must specify *either* ``choices`` or ``choice_loader`` op
 Example Usage
 -------------
 
-The easiest way to use this field is to specify the choices directly via
-the ``choices`` option::
+The easiest way to use this field is to define the ``choices`` option to specify
+the choices as an associative array where the keys are the labels displayed to
+end users and the array values are the internal values used in the form field::
 
     use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
     // ...