[SPARK-7572] [MLLIB] do not import Param/Params under pyspark.ml

Remove `Param` and `Params` from `pyspark.ml` and add a section in the doc. brkyvz

Author: Xiangrui Meng <meng@databricks.com>

Closes #6094 from mengxr/SPARK-7572 and squashes the following commits:

022abd6 [Xiangrui Meng] do not import Param/Params under spark.ml