[#2397] Adding more empty_data details from previous changes that were removed when merging into 2.1

Author: weaverryan

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

@@ -6,6 +6,17 @@ empty_data
 This option determines what value the field will return when the ``empty_value``
 choice is selected.
 
+The true default value of this option depends on the field options:
+
+* If ``compound`` is ``true`` and ``data_class`` is set, then ``new $data_class()``;
+* If ``compound`` is ``true`` and no ``data_class`` is set, then ``array()``;
+* If ``compound`` is ``false``, then ``null``.
+
+.. tip::
+
+    The ``compound`` option is set to ``true`` when the field actually represents
+    a collection of fields (e.g. a form of fields).
+
 For example, if you want the ``gender`` field to be set to ``null`` when no
 value is selected, you can do it like this: