[SPARK-14618][ML][DOC] Updated RegressionEvaluator.metricName param doc

## What changes were proposed in this pull request?

In Spark 1.4, we negated some metrics from RegressionEvaluator since CrossValidator always maximized metrics. This was fixed in 1.5, but the docs were not updated. This PR updates the docs.

## How was this patch tested?

no tests

Author: Joseph K. Bradley <[email protected]>

Closes #12377 from jkbradley/regeval-doc.

(cherry picked from commit bf65c87)
Signed-off-by: Joseph K. Bradley <[email protected]>

Author: jkbradley

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

@@ -36,11 +36,12 @@ final class RegressionEvaluator(override val uid: String)
   def this() = this(Identifiable.randomUID("regEval"))
 
   /**
-   * param for metric name in evaluation (supports `"rmse"` (default), `"mse"`, `"r2"`, and `"mae"`)
+   * Param for metric name in evaluation. Supports:
+   *  - `"rmse"` (default): root mean squared error
+   *  - `"mse"`: mean squared error
+   *  - `"r2"`: R^2^ metric
+   *  - `"mae"`: mean absolute error
    *
-   * Because we will maximize evaluation value (ref: `CrossValidator`),
-   * when we evaluate a metric that is needed to minimize (e.g., `"rmse"`, `"mse"`, `"mae"`),
-   * we take and output the negative of this metric.
    * @group param
    */
   val metricName: Param[String] = {