diff --git a/Doc/library/constants.rst b/Doc/library/constants.rst
--- a/Doc/library/constants.rst
+++ b/Doc/library/constants.rst
@@ -40,6 +40,13 @@ A small number of constants live in the built-in namespace.  They are:
    on the operator.  If all attempted operations return ``NotImplemented``, the
    interpreter will raise an appropriate exception.
 
+   Note that returning this value only makes sense for binary operations such as
+   :meth:`__lt__` or :meth:`__add__`. The appropriate way to signal that an
+   operation is unsupported is to leave the relevant method undefined. If the
+   method must be defined for some reason, invoking the default behaviour (by
+   calling the superclass' method with :func:`super`) or raising an exception
+   (typically :exc:`TypeError`) is the encouraged way to do this.
+
    See
    :ref:`implementing-the-arithmetic-operations`
    for more details.