Clarify the return value of __nonzero__(): It *must* be an integer.

Closes SF bug #579991.

Doc/lib/libstdtypes.tex

@@ -47,9 +47,9 @@ The following values are considered false:
 
 \item	instances of user-defined classes, if the class defines a
 	\method{__nonzero__()} or \method{__len__()} method, when that
-	method returns zero.\footnote{Additional information on these
-special methods may be found in the \citetitle[../ref/ref.html]{Python
-Reference Manual}.}
+	method returns the integer zero.\footnote{Additional
+information on these special methods may be found in the
+\citetitle[../ref/ref.html]{Python Reference Manual}.}
 
 \end{itemize}