[SPARK-12633][PYSPARK] [DOC] PySpark regression parameter desc to consistent format

Part of task for [SPARK-11219](https://issues.apache.org/jira/browse/SPARK-11219) to make PySpark MLlib parameter description formatting consistent. This is for the regression module.  Also, updated 2 params in classification to read as `Supported values:` to be consistent.

closes #10600

Author: vijaykiran <mail@vijaykiran.com>
Author: Bryan Cutler <cutlerb@gmail.com>

Closes #11404 from BryanCutler/param-desc-consistent-regression-SPARK-12633.