[SPARK-15168][PYSPARK][ML] Add missing params to MultilayerPerceptronClassifier

## What changes were proposed in this pull request?

MultilayerPerceptronClassifier is missing step size, solver, and weights. Add these params. Also clarify the scaladoc a bit while we are updating these params.

Eventually we should follow up and unify the HasSolver params (filed https://issues.apache.org/jira/browse/SPARK-15169 )

## How was this patch tested?

Doc tests

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

Closes #12943 from holdenk/SPARK-15168-add-missing-params-to-MultilayerPerceptronClassifier.