message: Add a comment to clarify subtle branch in node_copy_tree
diff: @@ -4423,6 +4423,7 @@ PUGI__NS_BEGIN
  while (sit && sit != sn)
  {
  + // when a tree is copied into one of the descendants, we need to skip that subtree to avoid an infinite loop
  if (sit != dn)
  {
  xml_node_struct* copy = append_new_node(dit, alloc, PUGI__NODETYPE(sit));
  