[SPARK-12630][PYSPARK] [DOC] PySpark classification 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 classification module.

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

Closes #11183 from BryanCutler/pyspark-consistent-param-classification-SPARK-12630.