Add description to Point_set tree example

Octree/doc/Octree/Octree.txt

@@ -18,7 +18,13 @@ The Octree package provides the Octree and Node classes.
 
 \subsection Section_Octree_Example_Point_Set Building an Octree from a Point_set_3
 
-e.g. I've loaded a `Point_set_3` from a file, and I want to build a tree from it.
+This example illustrates how to create an octree from a `Point_set_3` loaded from a file.
+
+An octree is constructed from the point set and its map.
+The tree is refined with a max-depth (deepest node allowed) of 10,
+and a bucket size (maximum number of points contained by a single node) of 20.
+The tree is then printed to the standard output.
+
 \cgalExample{Octree/Octree_build_from_Point_set.cpp}
 
 \subsection Section_Octree_Example_Point_Vector Building an Octree from a Vector of Points