[SPARK-15195][PYSPARK][DOCS] Update ml.tuning PyDocs

## What changes were proposed in this pull request?

Tag classes in ml.tuning as experimental, add docs for kfolds avg metric, and copy TrainValidationSplit scaladoc for more detailed explanation.

## How was this patch tested?

built docs locally

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

Closes #12967 from holdenk/SPARK-15195-pydoc-ml-tuning.