add precondition about single CC mesh

Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/approximated_centroidal_Voronoi_diagram_remeshing.h

@@ -1390,6 +1390,9 @@ acvd_impl(TriangleMesh& tmesh,
 *
 * \cgalNamedParamsEnd
 *
+*
+* @pre tmesh is a triangulated surface mesh and has exactly one connected component.
+*
 * @return `true` if `nb_vertices` was sufficiently large for remeshing the input, and `false` if more points were used
 *
 */

Surface_mesh_skeletonization/include/CGAL/extract_mean_curvature_flow_skeleton.h

@@ -43,6 +43,7 @@ namespace CGAL{
 ///        and the set of input vertices that contracted to `vd` can be retrieved
 ///        using `skeleton[vd].point` and `skeleton[vd].vertices` respectively.
 ///
+/// @pre `tmesh` is a triangulated surface mesh without borders and has exactly one connected component.
 template <class TriangleMesh>
 void extract_mean_curvature_flow_skeleton(const TriangleMesh& tmesh,
                                           typename Mean_curvature_flow_skeletonization<TriangleMesh>::Skeleton& skeleton)