[SPARK-9798] [ML] CrossValidatorModel Documentation Improvements

Document CrossValidatorModel members: bestModel and avgMetrics

Author: Rerngvit Yanggratoke <rerngvit@kth.se>

Closes #8882 from rerngvit/Spark-9798.

mllib/src/main/scala/org/apache/spark/ml/tuning/CrossValidator.scala

@@ -136,6 +136,10 @@ class CrossValidator(override val uid: String) extends Estimator[CrossValidatorM
 /**
  * :: Experimental ::
  * Model from k-fold cross validation.
+ *
+ * @param bestModel The best model selected from k-fold cross validation.
+ * @param avgMetrics Average cross-validation metrics for each paramMap in [[estimatorParamMaps]], in the
+ *                   corresponding order.
  */
 @Experimental
 class CrossValidatorModel private[ml] (