[SPARK-15189][PYSPARK][DOCS] Update ml.evaluation PyDoc

## What changes were proposed in this pull request?

Fix doctest issue, short param description, and tag items as Experimental

## How was this patch tested?

build docs locally & doctests

Author: Holden Karau <holden@us.ibm.com>

Closes #12964 from holdenk/SPARK-15189-ml.Evaluation-PyDoc-issues.