Added documentation

src/mlpack/methods/decision_tree/decision_tree.hpp

@@ -487,7 +487,10 @@ class DecisionTree :
 
   union
   {
+      //! Stores the type of dimension on which the split is done for internal
+      //! nodes of the tree.
       size_t dimensionType;
+      //! Stores the majority class for leaf nodes of the tree.
       size_t majorityClass;
   };
   /**

src/mlpack/methods/decision_tree/decision_tree_regressor.hpp

@@ -453,7 +453,9 @@ class DecisionTreeRegressor :
 
   union
   {
+      //! Stores the split point for internal nodes of the tree.
       double splitPoint;
+      //! Stores the prediction value for leaf nodes of the tree.
       double prediction;
   };