This PR adds the `relativeError` param to PySpark's `QuantileDiscretizer` to match Scala.

Also cleaned up a duplication of `numBuckets` where the param is both a class and instance attribute (I removed the instance attr to match the style of params throughout `ml`).

Finally, cleaned up the docs for `QuantileDiscretizer` to reflect that it now uses `approxQuantile`.

## How was this patch tested?

A little doctest and built API docs locally to check HTML doc generation.

Author: Nick Pentreath <nickp@za.ibm.com>

Closes #13228 from MLnick/SPARK-15442-py-relerror-param.