Documentation 
Section: treemodel.h, set_value_impl()
  //Called by TreeRow, which is a friend class:
  //The comment about set_row_changed() in the documentation is based on my reading of the source of
  //gtk_list_store_set_value() and gtk_tree_store_set_value().  
  /** Override and implement this in a derived TreeModel class, so that Row::operator() and
   * Row::set_value() work.
   * You can probably just implement this by calling set_value_vfunc().
   * Your implementation of set_value_impl() should also call set_row_changed() after changing the value.
   */

Correct version:
  //Called by TreeRow, which is a friend class:
  //The comment about row_changed() in the documentation is based on my reading of the source of
  //gtk_list_store_set_value() and gtk_tree_store_set_value().  
  /** Override and implement this in a derived TreeModel class, so that Row::operator() and
   * Row::set_value() work.
   * You can probably just implement this by calling set_value_vfunc().
   * Your implementation of set_value_impl() should also call row_changed() after changing the value.
   */

Other information:
Just a typo. There's no set_row_changed() function in gtkmm, only row_changed().