[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
If we decide to cache, let's note why we are doing it via a comment. Else, some users might get confused and decide to cache always before calling the tree algorithm.