Fixed #16291 -- Documented that TypedChoiceField does not coerce empty_value.…

Fixed #16291 -- Documented that TypedChoiceField does not coerce empty_value. Thanks to vanschelven and taavi223.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16820 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/forms/fields.txt

@@ -379,7 +379,9 @@ Takes extra arguments:
 .. attribute:: TypedChoiceField.empty_value
 
     The value to use to represent "empty." Defaults to the empty string;
-    ``None`` is another common choice here.
+    ``None`` is another common choice here. Note that this value will not be
+    coerced by the function given in the ``coerce`` argument, so choose it
+    accordingly.
 
 ``DateField``
 ~~~~~~~~~~~~~