add warning about exception messages
diff --git a/Doc/ref/ref4.tex b/Doc/ref/ref4.tex
index 6e893f1..00f3058 100644
--- a/Doc/ref/ref4.tex
+++ b/Doc/ref/ref4.tex
@@ -197,5 +197,12 @@
exception handler as additional information. For class exceptions,
this object must be an instance of the exception class being raised.
+\begin{notice}[warning]
+Messages to exceptions are not part of the Python API. Their contents may
+change from one version of Python to the next without warning and should not
+be relied on by code which will run under multiple versions of the
+interpreter.
+\end{notice}
+
See also the description of the \keyword{try} statement in section
\ref{try} and \keyword{raise} statement in section \ref{raise}.