Simplify the production for argument list, making sure that it

actually allows all the legal syntax, and nothing else.  Previously,
it did not allow a call like func(arg, **dictionary).
This closes (again!) SF bug #493243.

Doc/ref/ref5.tex

@@ -422,10 +422,10 @@ series of arguments:
   \production{call}
              {\token{primary} "(" [\token{argument_list} [","]] ")"}
   \production{argument_list}
-             {\token{positional_arguments} ["," \token{keyword_arguments}
-              ["," "*" \token{expression} ["," "**" \token{expression}]]]}
-  \productioncont{| \token{keyword_arguments} ["," "*" \token{expression}
-                                               ["," "**" \token{expression}]]}
+             {\token{positional_arguments} ["," \token{keyword_arguments}]
+              ["," "*" \token{expression}] ["," "**" \token{expression}]}
+  \productioncont{| \token{keyword_arguments} ["," "*" \token{expression}]
+                                               ["," "**" \token{expression}]}
   \productioncont{| "*" \token{expression} ["," "**" \token{expression}]}
   \productioncont{| "**" \token{expression}}
   \production{positional_arguments}