document overriden options of checkbox

Author: nykopol

reference/forms/types/options/checkbox_compound.rst.inc

@@ -3,4 +3,6 @@ compound
 
 **type**: ``boolean`` **default**: ``false``
 
-Specify that checkbox type is not a combined type.
+This option specifies if a form is compound. As it's not the
+case for checkbox, by fefault the value is overriden with
+``false`` value.

reference/forms/types/options/checkbox_empty_data.rst.inc

@@ -1,6 +1,10 @@
 empty_data
 ~~~~~~~~~~
 
-**type**: ``closure`` 
+**type**: ``string`` **default**: ``mixed``
 
+This option determines what value the field will return when the ``empty_value``
+choice is selected.
+In checkbox, the value of ``empty_data`` is overriden by the value returned by
+the data transformer (see :doc:`/cookbook/form/data_transformers.rst`).