code: public ClassDoc superclass() {
          if (isInterface() || isAnnotationType()) return null;
          if (tsym == env.syms.objectType.tsym) return null;
          ClassSymbol c = (ClassSymbol)env.types.supertype(type).tsym;
          if (c == null || c == tsym) c = (ClassSymbol)env.syms.objectType.tsym;
          return env.getClassDoc(c);
      }
docs: Return the superclass of this class
  
  @return the ClassDocImpl for the superclass of this class, null
  if there is no superclass.