[SPARK-11602][MLLIB] Refine visibility for 1.6 scala API audit

jira: https://issues.apache.org/jira/browse/SPARK-11602

Made a pass on the API change of 1.6. Open the PR for efficient discussion.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes apache#9939 from hhbyyh/auditScala.

Author: hhbyyh

mllib/src/main/scala/org/apache/spark/ml/feature/package-info.java

@@ -23,7 +23,7 @@
  * features into more suitable forms for model fitting.
  * Most feature transformers are implemented as {@link org.apache.spark.ml.Transformer}s, which
  * transforms one {@link org.apache.spark.sql.DataFrame} into another, e.g.,
- * {@link org.apache.spark.feature.HashingTF}.
+ * {@link org.apache.spark.ml.feature.HashingTF}.
  * Some feature transformers are implemented as {@link org.apache.spark.ml.Estimator}}s, because the
  * transformation requires some aggregated information of the dataset, e.g., document
  * frequencies in {@link org.apache.spark.ml.feature.IDF}.

mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala

@@ -529,7 +529,7 @@ class LinearRegressionSummary private[regression] (
     val predictionCol: String,
     val labelCol: String,
     val model: LinearRegressionModel,
-    val diagInvAtWA: Array[Double]) extends Serializable {
+    private val diagInvAtWA: Array[Double]) extends Serializable {
 
   @transient private val metrics = new RegressionMetrics(
     predictions

mllib/src/main/scala/org/apache/spark/mllib/clustering/BisectingKMeans.scala

@@ -407,7 +407,7 @@ private object BisectingKMeans extends Serializable {
  */
 @Since("1.6.0")
 @Experimental
-class ClusteringTreeNode private[clustering] (
+private[clustering] class ClusteringTreeNode private[clustering] (
     val index: Int,
     val size: Long,
     private val centerWithNorm: VectorWithNorm,

mllib/src/main/scala/org/apache/spark/mllib/clustering/BisectingKMeansModel.scala

@@ -32,8 +32,8 @@ import org.apache.spark.rdd.RDD
  */
 @Since("1.6.0")
 @Experimental
-class BisectingKMeansModel @Since("1.6.0") (
-    @Since("1.6.0") val root: ClusteringTreeNode
+class BisectingKMeansModel private[clustering] (
+    private[clustering] val root: ClusteringTreeNode
   ) extends Serializable with Logging {
 
   /**