minor: added doc for maxMemory parameter

Author: manishamde

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

@@ -35,6 +35,9 @@ 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 maxMemory maximum memory in MB allocated to histogram aggregation. Default value is
+ *                  128 MB.
+ *
  */
 @Experimental
 class Strategy (