clarify a bit of Variables doxy

libwccl/variables.h

@@ -77,7 +77,7 @@ public:
  * Values are subtypes of the Value class, valid types are listed as arguments
  * of instantiations of the detail::Vmap base class template, and in the type
  * list below. Adding a new subtype should only take adding it to these two
- * places.
+ * places: inherit from another Vmap, add to typelist.
  */
 class Variables : detail::Vmap<Value>
 	, detail::Vmap<Bool>