[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

Author: Joseph K. Bradley <joseph.kurata.bradley@gmail.com>

Closes #2063 from jkbradley/dt-docs and squashes the following commits:

2dd2c19 [Joseph K. Bradley] Last updates based on github review.
9dd1b6b [Joseph K. Bradley] Updated decision tree doc.
d802369 [Joseph K. Bradley] Updates based on comments: cache data, corrected doc text.
b9bee04 [Joseph K. Bradley] Updated DT examples
57eee9f [Joseph K. Bradley] Created JavaDecisionTree example from example in docs, and corrected doc example as needed.
d939a92 [Joseph K. Bradley] Updated DecisionTree documentation.  Added Java, Python examples.
Merge branch 'SPARK-3171' of github.com:sarutak/spark into SPARK-3106
Modified closing order of channel and selector to avoid unwilling CancelledKeyException
Modified Connection.scala and ConnectionManager.scala to avoid calling channel.close double by other 2 threads