[SPARK-19590][PYSPARK][ML] Update the document for QuantileDiscretizer in pyspark

## What changes were proposed in this pull request?
This PR is to document the changes on QuantileDiscretizer in pyspark for PR:
https://github.com/apache/spark/pull/15428

## How was this patch tested?
No test needed

Signed-off-by: VinceShieh <vincent.xieintel.com>

Author: VinceShieh <vincent.xie@intel.com>

Closes #16922 from VinceShieh/spark-19590.