xml::Optional example added end extended

src/xml-cxx/xml.hxx

@@ -165,7 +165,7 @@ class MethodTrace {
        - most standard containers, but in their xml-form,
          e.g. xml::List instead of @c std::list
          (xml::List inherits @c std::list)
-       - Optional values through xml::Optional
+       - Optional values are supported through xml::Optional
     - @c std::bitset, @c std::priority_queue, @c std::queue and
       @c std::stack are not implemented
     - Polymorfic serialisation is not yet implemented
@@ -1276,7 +1276,8 @@ namespace xml {
       interfaces to another container and don't allow random access
       (which is needed to store them).
 
-      @example list_serialization.cxx */
+      @example list_serialization.cxx
+      @example optional_serialization.cxx */
   //@}
 
   //! @cond INTERNAL