[SPARK-12368][ML][DOC] Better doc for the binary classification evaluator' metricName

For the BinaryClassificationEvaluator, the scaladoc doesn't mention that "areaUnderPR" is supported, only that the default is "areadUnderROC".
Also, in the documentation, it is said that:
"The default metric used to choose the best ParamMap can be overriden by the setMetric method in each of these evaluators."
However, the method is called setMetricName.

This PR aims to fix both issues.

Author: BenFradet <benjamin.fradet@gmail.com>

Closes #10328 from BenFradet/SPARK-12368.

docs/ml-guide.md

@@ -628,7 +628,7 @@ Currently, `spark.ml` supports model selection using the [`CrossValidator`](api/
 The `Evaluator` can be a [`RegressionEvaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.RegressionEvaluator)
 for regression problems, a [`BinaryClassificationEvaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.BinaryClassificationEvaluator)
 for binary data, or a [`MultiClassClassificationEvaluator`](api/scala/index.html#org.apache.spark.ml.evaluation.MultiClassClassificationEvaluator)
-for multiclass problems. The default metric used to choose the best `ParamMap` can be overriden by the `setMetric`
+for multiclass problems. The default metric used to choose the best `ParamMap` can be overriden by the `setMetricName`
 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.
@@ -951,4 +951,4 @@ model.transform(test)
 {% endhighlight %}
 </div>
 
-</div>
\ No newline at end of file
+</div>

mllib/src/main/scala/org/apache/spark/ml/evaluation/BinaryClassificationEvaluator.scala

@@ -39,8 +39,7 @@ class BinaryClassificationEvaluator @Since("1.4.0") (@Since("1.4.0") override va
   def this() = this(Identifiable.randomUID("binEval"))
 
   /**
-   * param for metric name in evaluation
-   * Default: areaUnderROC
+   * param for metric name in evaluation (supports `"areaUnderROC"` (default), `"areaUnderPR"`)
    * @group param
    */
   @Since("1.2.0")