`make clean html` under `python/doc` returns
~~~
/Users/meng/src/spark/python/pyspark/ml/evaluation.py:docstring of pyspark.ml.evaluation.RegressionEvaluator.setParams:3: WARNING: Definition list ends without a blank line; unexpected unindent.
~~~

harsha2010

Author: Xiangrui Meng <meng@databricks.com>

Closes #6469 from mengxr/fix-regression-evaluator-doc and squashes the following commits:

91e2dad [Xiangrui Meng] fix RegressionEvaluator doc

(cherry picked from commit 834e6995)
Signed-off-by: Xiangrui Meng <meng@databricks.com>