[1.6.x] Added release note for TypedChoiceField coerce limitation

Thanks Elec for the report and Simon Charette for the review.
Refs #21397.
Backport of 4a00f132e0 from master.

docs/releases/1.6.txt

@@ -873,6 +873,11 @@ Miscellaneous
   :class:`~django.forms.SlugField` have been suppressed because they
   duplicated error messages already provided by validators tied to the fields.
 
+* Due to a change in the form validation workflow,
+  :class:`~django.forms.TypedChoiceField` ``coerce`` method should always
+  return a value present in the ``choices`` field attribute. That limitation
+  should be lift again in Django 1.7.
+
 * There have been changes in the way timeouts are handled in cache backends.
   Explicitly passing in ``timeout=None`` no longer results in using the
   default timeout. It will now set a non-expiring timeout. Passing 0 into the