Add documentation for element type.
Add documentation for element type.

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