clarify trailing comma in function argument list
(SF bug #798652)
diff --git a/Doc/ref/ref5.tex b/Doc/ref/ref5.tex
index c7e5fd7..036533c 100644
--- a/Doc/ref/ref5.tex
+++ b/Doc/ref/ref5.tex
@@ -493,8 +493,8 @@
{\token{identifier} "=" \token{expression}}
\end{productionlist}
-A trailing comma may be present after an argument list but does not
-affect the semantics.
+A trailing comma may be present after the positional and keyword
+arguments but does not affect the semantics.
The primary must evaluate to a callable object (user-defined
functions, built-in functions, methods of built-in objects, class