[DOCS] Fix Gini and Entropy scaladocs in context of multiclass classification

The PR changes outdated scaladocs for Gini and Entropy classes. Since PR #886 Spark supports multiclass classification, but the docs tell only about binary classification.

Author: Wojciech Jurczyk <wojciech.jurczyk@codilime.com>

Closes #11252 from wjur/wjur/docs_multiclass.

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Entropy.scala

@@ -21,8 +21,7 @@ import org.apache.spark.annotation.{DeveloperApi, Experimental, Since}
 
 /**
  * :: Experimental ::
- * Class for calculating [[http://en.wikipedia.org/wiki/Binary_entropy_function entropy]] during
- * binary classification.
+ * Class for calculating entropy during multiclass classification.
  */
 @Since("1.0.0")
 @Experimental

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Gini.scala

@@ -23,7 +23,7 @@ import org.apache.spark.annotation.{DeveloperApi, Experimental, Since}
  * :: Experimental ::
  * Class for calculating the
  * [[http://en.wikipedia.org/wiki/Decision_tree_learning#Gini_impurity Gini impurity]]
- * during binary classification.
+ * during multiclass classification.
  */
 @Since("1.0.0")
 @Experimental