Clarify that the bool instances are acceptable return values from

__nonzero__(), in response to SF bug #579991.

Doc/lib/libstdtypes.tex

@@ -47,7 +47,8 @@ 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 the integer zero.\footnote{Additional
+        method returns the integer zero or \class{bool} value
+        \code{False}.\footnote{Additional 
 information on these special methods may be found in the
 \citetitle[../ref/ref.html]{Python Reference Manual}.}