Document the NotImplemented object.

Doc/ref/ref3.tex

@@ -130,6 +130,15 @@ Its truth value is false.
 \ttindex{None}
 \obindex{None@{\texttt{None}}}
 
+\item[NotImplemented]
+This type has a single value.  There is a single object with this value.
+This object is accessed through the built-in name \code{NotImplemented}.
+Binary number methods may return this value if they do not implement the
+operation for the types of operands provided.  The interpreter will then
+try the reverse operation. Its truth value is true.
+\ttindex{NotImplemented}
+\obindex{NotImplemented@{\texttt{NotImplemented}}}
+
 \item[Ellipsis]
 This type has a single value.  There is a single object with this value.
 This object is accessed through the built-in name \code{Ellipsis}.