Mention that type.__prepare__() exists for use via super().

pep-3115.txt

@@ -132,6 +132,10 @@ Invoking the Metaclass
      assignment algorithm. (Note also that 'metaclass' is a keyword-
      only argument as per PEP 3102 [6].)
 
+     Even though __prepare__ is not required, the default metaclass
+     ('type') implements it, for the convenience of subclasses calling
+     it via super().
+
      __prepare__ returns a dictionary-like object which is used to store
      the class member definitions during evaluation of the class body.
      In other words, the class body is evaluated as a function block