SF bug #661848 and #631055: Clarify use of __all__.
diff --git a/Doc/ref/ref6.tex b/Doc/ref/ref6.tex
index 9c088e9..6c252ed 100644
--- a/Doc/ref/ref6.tex
+++ b/Doc/ref/ref6.tex
@@ -702,7 +702,10 @@
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{_}).
+(\character{_}). \code{__all__} should contain the entire public API.
+It is intended to avoid accidentally exporting items that are not part
+of the API (such as library modules which were imported and used within
+the module).
\withsubitem{(optional module attribute)}{\ttindex{__all__}}
The \keyword{from} form with \samp{*} may only occur in a module