[SPARK-4685] Include all spark.ml and spark.mllib packages in JavaDoc's MLlib group

This is #3554 from Lewuathe except that I put both `spark.ml` and `spark.mllib` in the group 'MLlib`.

Closes #3554

jkbradley

Author: lewuathe <lewuathe@me.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #3598 from mengxr/Lewuathe-modify-javadoc-setting and squashes the following commits:

184609a [Xiangrui Meng] merge spark.ml and spark.mllib into the same group in javadoc
f7535e6 [lewuathe] [SPARK-4685] Update JavaDoc settings to include spark.ml and all spark.mllib subpackages in the right sections

(cherry picked from commit 20bfea4ab7c0923e8d3f039d0c5098669db4d5b0)
Signed-off-by: Xiangrui Meng <meng@databricks.com>