[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 are only calculating numClasses in the Java example. Should we eliminate it since it makes the already verbose Java code even more verbose? Else, we need to make the same change to the Scala and Python examples.