Better explain empty_data values

Author: wouterj

form/use_empty_data.rst

@@ -27,7 +27,14 @@ a ``data_class`` option for your form class, it will default to a new instance
 of that class. That instance will be created by calling the constructor
 with no arguments.
 
-If you want to override this default behavior, there are two ways to do this.
+If you want to override this default behavior, there are two ways to do this:
+
+* `Option 1: Instantiate a new Class`_
+* `Option 2: Provide a Closure`_
+
+If you didn't set the ``data_class`` option, you can pass the initial data as
+string or pass an array of strings (where the key matches the field name) when
+the form type is compound.
 
 Option 1: Instantiate a new Class
 ---------------------------------

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

@@ -26,9 +26,9 @@ This will still render an empty text box, but upon submission the ``John Doe``
 value will be set. Use the ``data`` or ``placeholder`` options to show this
 initial value in the rendered form.
 
-If a form is compound, you can set ``empty_data`` as an array with field names
-as keys and submitted values as string values (or arrays if nested fields are
-also compound).
+If a form is compound, you can set ``empty_data`` as an array, object or
+closure. See the :doc:`/form/use_empty_data` article for more details about
+these options.
 
 .. note::