clarify trailing comma in function argument list

(SF bug #798652)

Doc/ref/ref5.tex

@@ -493,8 +493,8 @@ series of arguments:
              {\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

Doc/ref/ref7.tex

@@ -323,9 +323,9 @@ section~\ref{types}):
              {"@" \token{dotted_name} ["(" [\token{argument_list} [","]] ")"] NEWLINE}
   \production{parameter_list}
                  {(\token{defparameter} ",")*}
-  \productioncont{("*" \token{identifier} [, "**" \token{identifier}]}
-  \productioncont{| "**" \token{identifier}
-                  | \token{defparameter} [","])}
+  \productioncont{(~~"*" \token{identifier} [, "**" \token{identifier}]}
+  \productioncont{ | "**" \token{identifier}}
+  \productioncont{ | \token{defparameter} [","] )}
   \production{defparameter}
              {\token{parameter} ["=" \token{expression}]}
   \production{sublist}