[SPARK-9911] [DOC] [ML] Update Userguide for Evaluator

I added a small note about the different types of evaluator and the metrics used.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #8304 from MechCoder/multiclass_evaluator.

docs/ml-guide.md

@@ -643,6 +643,13 @@ An important task in ML is *model selection*, or using data to find the best mod
 Currently, `spark.ml` supports model selection using the [`CrossValidator`](api/scala/index.html#org.apache.spark.ml.tuning.CrossValidator) class, which takes an `Estimator`, a set of `ParamMap`s, and an [`Evaluator`](api/scala/index.html#org.apache.spark.ml.Evaluator).
 `CrossValidator` begins by splitting the dataset into a set of *folds* which are used as separate training and test datasets; e.g., with `$k=3$` folds, `CrossValidator` will generate 3 (training, test) dataset pairs, each of which uses 2/3 of the data for training and 1/3 for testing.
 `CrossValidator` iterates through the set of `ParamMap`s. For each `ParamMap`, it trains the given `Estimator` and evaluates it using the given `Evaluator`.
+
+The `Evaluator` can be a [`RegressionEvaluator`](api/scala/index.html#org.apache.spark.ml.RegressionEvaluator)
+for regression problems, a [`BinaryClassificationEvaluator`](api/scala/index.html#org.apache.spark.ml.BinaryClassificationEvaluator)
+for binary data or a [`MultiClassClassificationEvaluator`](api/scala/index.html#org.apache.spark.ml.MultiClassClassificationEvaluator)
+for multiclass problems. The default metric used to choose the best `ParamMap` can be overriden by the setMetric
+method in each of these evaluators.
+
 The `ParamMap` which produces the best evaluation metric (averaged over the `$k$` folds) is selected as the best model.
 `CrossValidator` finally fits the `Estimator` using the best `ParamMap` and the entire dataset.
 
@@ -708,9 +715,12 @@ val pipeline = new Pipeline()
 // We now treat the Pipeline as an Estimator, wrapping it in a CrossValidator instance.
 // This will allow us to jointly choose parameters for all Pipeline stages.
 // A CrossValidator requires an Estimator, a set of Estimator ParamMaps, and an Evaluator.
+// Note that the evaluator here is a BinaryClassificationEvaluator and the default metric
+// used is areaUnderROC.
 val crossval = new CrossValidator()
   .setEstimator(pipeline)
   .setEvaluator(new BinaryClassificationEvaluator)
+
 // We use a ParamGridBuilder to construct a grid of parameters to search over.
 // With 3 values for hashingTF.numFeatures and 2 values for lr.regParam,
 // this grid will have 3 x 2 = 6 parameter settings for CrossValidator to choose from.
@@ -831,9 +841,12 @@ Pipeline pipeline = new Pipeline()
 // We now treat the Pipeline as an Estimator, wrapping it in a CrossValidator instance.
 // This will allow us to jointly choose parameters for all Pipeline stages.
 // A CrossValidator requires an Estimator, a set of Estimator ParamMaps, and an Evaluator.
+// Note that the evaluator here is a BinaryClassificationEvaluator and the default metric
+// used is areaUnderROC.
 CrossValidator crossval = new CrossValidator()
     .setEstimator(pipeline)
     .setEvaluator(new BinaryClassificationEvaluator());
+
 // We use a ParamGridBuilder to construct a grid of parameters to search over.
 // With 3 values for hashingTF.numFeatures and 2 values for lr.regParam,
 // this grid will have 3 x 2 = 6 parameter settings for CrossValidator to choose from.