Update documentation for constant()

doc/functions/constant.rst

@@ -1,9 +1,18 @@
 ``constant``
 ============
 
+.. versionadded: 1.13
+    constant() now accepts object instances as the second argument.
+
 ``constant`` returns the constant value for a given string:
 
 .. code-block:: jinja
 
     {{ some_date|date(constant('DATE_W3C')) }}
     {{ constant('Namespace\\Classname::CONSTANT_NAME') }}
+
+As of 1.13 you can read constants from object instances as well:
+
+.. code-block:: jinja
+
+    {{ constant('RSS', date) }}