Fixed #21397 -- Re-added flexibility to TypedChoiceField coercion

Thanks Elec for the report and Simon Charette for the review.

docs/ref/forms/fields.txt

@@ -375,7 +375,9 @@ For each field, we describe the default widget used if you don't specify
 
         A function that takes one argument and returns a coerced value. Examples
         include the built-in ``int``, ``float``, ``bool`` and other types. Defaults
-        to an identity function.
+        to an identity function. Note that coercion happens after input
+        validation, so it is possible to coerce to a value not present in
+        ``choices``.
 
     .. attribute:: empty_value
 

docs/releases/1.7.txt

@@ -317,6 +317,10 @@ Forms
   return ``self.cleaned_data``. If it does return a changed dictionary then
   that will still be used.
 
+* After a temporary regression in Django 1.6, it's now possible again to make
+  :class:`~django.forms.TypedChoiceField` ``coerce`` method return an arbitrary
+  value.
+
 * :attr:`SelectDateWidget.months
   <django.forms.extras.widgets.SelectDateWidget.months>` can be used to
   customize the wording of the months displayed in the select widget.