add documentation of parameters

Mesh_3/doc/Mesh_3/CGAL/Gray_image_mesh_domain_3.h

@@ -53,8 +53,13 @@ public:
 /// @{
 
 /*!
-Construction from an image. 
-The parameter `error_bound` is relative to the size of the image. 
+Construction from an image.
+@param image the input image
+@param iso_value the isovalue, inside `image`,
+       of the surface describing the boundary of the object to be meshed
+@param value_outside the value attached to voxels outside of the domain
+       to be meshed
+@param error_bound is relative to the size of the image. 
 */ 
   Gray_image_mesh_domain_3(
       const Image& image,

Mesh_3/doc/Mesh_3/PackageDescription.txt

@@ -98,6 +98,7 @@ and their associated classes:
 - `CGAL::Polyhedral_mesh_domain_3<Polyhedron,IGT,TriangleAccessor>`
 - `CGAL::Polyhedral_mesh_domain_with_features_3<IGT>`
 - `CGAL::Labeled_image_mesh_domain_3<Image,BGT>`
+- `CGAL::Gray_image_mesh_domain_3<Image,BGT,Image_word_type>`
 - `CGAL::Mesh_domain_with_polyline_features_3<MeshDomain_3>`
 - `CGAL::Mesh_polyhedron_3<IGT>`
 - `CGAL::Triangle_accessor_3<CGAL::Polyhedron_3<K>,K>`