add api docs

Author: qiping.lqp

mllib/src/main/scala/org/apache/spark/mllib/tree/configuration/Strategy.scala

@@ -49,6 +49,12 @@ import org.apache.spark.mllib.tree.configuration.QuantileStrategy._
  *                                k) implies the feature n is categorical with k categories 0,
  *                                1, 2, ... , k-1. It's important to note that features are
  *                                zero-indexed.
+ * @param minInstancesPerNode Minimum number of nodes each child must have after split. Default value is 0.
+ *                            If a split cause left or right child to have less than minInstancesPerNode,
+ *                            this split will not be considered as a valid split.
+ * @param minInfoGain Minimum information gain a split must get. Default value is 0.0.
+ *                    If a split has less information gain than minInfoGain,
+ *                    this split will not be considered as a valid split.
  * @param maxMemoryInMB Maximum memory in MB allocated to histogram aggregation. Default value is
  *                      128 MB.
  */