make is_same's all-important enum appear in the doxygen documentation

[[Imported from SVN: r5555]]

common/typetraits.hh

@@ -295,6 +295,7 @@ namespace Dune
   template<typename T1, typename T2>
   struct is_same
   {
+    //! Whether T1 is the same type as T2.
     enum {
       /* @brief Whether T1 is the same type as T2. */
       value=false