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

(cherry picked from commit 67cc89ff028324ba4a7a7d9c19a268b9afea0031)
Signed-off-by: Nick Pentreath <6fe12bfdafd6d3c41da5ad6593c53ed449c85282@za.ibm.com>

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