Document the third template parameter

Apollonius_graph_2/doc/Apollonius_graph_2/CGAL/Apollonius_graph_vertex_base_2.h

@@ -7,23 +7,30 @@ namespace CGAL {
 The class `Apollonius_graph_vertex_base_2` provides a model for the
 `ApolloniusGraphVertexBase_2` concept which is the vertex base
 required by the `ApolloniusGraphDataStructure_2` concept. The
-class `Apollonius_graph_vertex_base_2` has two template arguments, the first being the
-geometric traits of the Apollonius graph and should be a model of the
-concept `ApolloniusGraphTraits_2`. The second is a Boolean which
+class `Apollonius_graph_vertex_base_2` has two template arguments.
+
+\tparam Gt is the geometric traits of the Apollonius graph and must be a model of the
+concept `ApolloniusGraphTraits_2`.
+
+\tparam StoreHidden is a Boolean which
 controls whether hidden sites are actually stored. Such a
 control is important if the user is not interested in hidden sites
 and/or if only insertions are made, in which case no hidden
 site can become visible. If `StoreHidden` is set to
 `true`, hidden sites are stored, otherwise they are
 discarded. By default `StoreHidden` is set to `true`.
 
+\tparam Vb must be a model of the concept `TriangulationDSVertexBase_2`
+By default this parameter is
+instantiated by `Triangulation_ds_vertex_base_2<>`.
+
 \cgalModels `ApolloniusGraphVertexBase_2`
 
 \sa `CGAL::Triangulation_data_structure_2<Vb,Fb>`
 \sa `CGAL::Apollonius_graph_hierarchy_vertex_base_2<Gt>`
 */
-template< typename Gt, typename StoreHidden >
-class Apollonius_graph_vertex_base_2 {
+  template< typename Gt, typename StoreHidden, typename Vb >
+  class Apollonius_graph_vertex_base_2 : public Vb {
 public:
 
 /// \name Creation