Add documentation for new backtrace functionality.
Add documentation for new backtrace functionality.

ajjl/mlpack,darcyliu/mlpack,ajjl/mlpack,darcyliu/mlpack,ranjan1990/mlpack,ranjan1990/mlpack,palashahuja/mlpack,palashahuja/mlpack,palashahuja/mlpack,ranjan1990/mlpack,ajjl/mlpack,darcyliu/mlpack