improve comment

From-SVN: r39806

gcc/cp/call.c

@@ -5212,7 +5212,12 @@ joust (cand1, cand2, warn)
       winner = more_specialized
         (TI_TEMPLATE (cand1->template), TI_TEMPLATE (cand2->template),
          DEDUCE_ORDER,
-         /* Never do unification on the 'this' parameter.  */
+         /* Tell the deduction code how many real function arguments we saw,
+	    not counting the implicit 'this' argument.
+
+	    [temp.func.order]: The presence of unused ellipsis and default
+	    arguments has no effect on the partial ordering of function
+	    templates.  */
          TREE_VEC_LENGTH (cand1->convs)
          - DECL_NONSTATIC_MEMBER_FUNCTION_P (cand1->fn));
       if (winner)