Added extra hint about status flags when collapsing edges

git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@1211 fdac6126-5c0c-442c-9429-916003d36597

Doc/operations.docu

@@ -78,6 +78,11 @@ A simple code example related to the illustration might look like this:
 \code
 PolyMesh mesh;
 
+// You need all status attributes to do delete operations
+mesh.request_edge_status();
+mesh.request_vertex_status();
+mesh.request_face_status();
+
 // Add some vertices as in the illustration above
 PolyMesh::VertexHandle vhandle[7];