Revise the __all__ index entry a touch.

Doc/ref/ref6.tex

@@ -703,7 +703,7 @@ considered public and are required to exist.  If \code{__all__} is not
 defined, the set of public names includes all names found in the
 module's namespace which do not begin with an underscore character
 (\character{_}).
-\index{__all__}
+\withsubitem{(optional module attribute)}{\ttindex{__all__}}
 
 The \keyword{from} form with \samp{*} may only occur in a module
 scope.  If the wild card form of import --- \samp{import *} --- is