[SPARK-16153][MLLIB] switch to multi-line doc to avoid a genjavadoc bug

## What changes were proposed in this pull request?

We recently deprecated setLabelCol in ChiSqSelectorModel (#13823):

~~~scala
  /** group setParam */
  Since("1.6.0")
  deprecated("labelCol is not used by ChiSqSelectorModel.", "2.0.0")
  def setLabelCol(value: String): this.type = set(labelCol, value)
~~~

This unfortunately hit a genjavadoc bug and broken doc generation. This is the generated Java code:

~~~java
  /** group setParam */
  public  org.apache.spark.ml.feature.ChiSqSelectorModel setOutputCol (java.lang.String value)  { throw new RuntimeException(); }
   *
   * deprecated labelCol is not used by ChiSqSelectorModel. Since 2.0.0.
  */
  public  org.apache.spark.ml.feature.ChiSqSelectorModel setLabelCol (java.lang.String value)  { throw new RuntimeException(); }
~~~

Switching to multiline is a workaround.

Author: Xiangrui Meng <meng@databricks.com>

Closes #13855 from mengxr/SPARK-16153.

mllib/src/main/scala/org/apache/spark/ml/feature/ChiSqSelector.scala

@@ -140,7 +140,9 @@ final class ChiSqSelectorModel private[ml] (
   @Since("1.6.0")
   def setOutputCol(value: String): this.type = set(outputCol, value)
 
-  /** @group setParam */
+  /**
+   * @group setParam
+   */
   @Since("1.6.0")
   @deprecated("labelCol is not used by ChiSqSelectorModel.", "2.0.0")
   def setLabelCol(value: String): this.type = set(labelCol, value)