[jvm-packages] Spark Scaladoc Value Member Groups

Currently we have no grouping for value members of classes like `XGBoostClassificationModel` and `XGBoostRegressionModel`, this makes the docs quite hard to look at:


If we compare this with the official spark docs, you can see groupings for things like 'paramaters', 'getters/setters', etc: [GBTClassificationModel Scala Docs](https://spark.apache.org/docs/2.4.3/api/scala/index.html#org.apache.spark.ml.classification.GBTClassificationModel)

Adding this is quite easy, but a little tedious, you need to add `@group` decorators like the following: https://github.com/apache/spark/blob/v2.4.3/mllib/src/main/scala/org/apache/spark/ml/classification/GBTClassifier.scala#L70