[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
That sounds a bit detailed for this overview, but it could be interesting for the blog post.  I'm about to push an update with more comments on maxBins, which is related to this but more about how the user should set parameters.