code: protected void addTree(SortedSet<TypeElement> sset, String heading, HtmlTree div) {
          addTree(sset, heading, div, false);
      }
docs: Add the heading for the tree depending upon tree type if it's a
  Class Tree or Interface tree.
  
  @param sset classes which are at the most base level, all the
  other classes in this run will derive from these classes
  @param heading heading for the tree
  @param div the content tree to which the tree will be added