mention how to override boolean evaluation

Doc/reference/expressions.rst

@@ -1143,7 +1143,8 @@ In the context of Boolean operations, and also when expressions are used by
 control flow statements, the following values are interpreted as false:
 ``False``, ``None``, numeric zero of all types, and empty strings and containers
 (including strings, tuples, lists, dictionaries, sets and frozensets).  All
-other values are interpreted as true.
+other values are interpreted as true.  (See the :meth:`~object.__nonzero__`
+special method for a way to change this.)
 
 .. index:: operator: not