[SPARK-15587][ML] ML 2.0 QA: Scala APIs audit for ml.feature

## What changes were proposed in this pull request?
ML 2.0 QA: Scala APIs audit for ml.feature. Mainly include:
* Remove seed for ```QuantileDiscretizer```, since we use ```approxQuantile``` to produce bins and ```seed``` is useless.
* Scala API docs update.
* Sync Scala and Python API docs for these changes.

## How was this patch tested?
Exist tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #13410 from yanboliang/spark-15587.