[SPARK-20047][FOLLOWUP][ML] Constrained Logistic Regression follow up

## What changes were proposed in this pull request?
Address some minor comments for #17715:
* Put bound-constrained optimization params under expertParams.
* Update some docs.

## How was this patch tested?
Existing tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #17829 from yanboliang/spark-20047-followup.