[SPARK-2840] [mllib] DecisionTree doc update (Java, Python examples)

Updated DecisionTree documentation, with examples for Java, Python.
Added same Java example to code as well.
CC: @mengxr  @manishamde @atalwalkar
We cached the binned features in training. But in this example, we visit the raw features twice. Since it is reading from disk, it should help if we cache the data.