title: cross validation function crossvalind
body: <p>I have question please; concerning cross validation, for me the cross-validation is used to find the best parameters.
  but I did not understand the role of this function "crossvalind":Generate cross-validation indices, it just takes a data set without model, like in this exemple : </p>
  
  <pre><code>load fisheriris
  [g gn] = grp2idx(species);                     
  [trainIdx testIdx] = crossvalind('HoldOut', species, 1/3);
  </code></pre>