## What changes were proposed in this pull request?
Updates user guide to reflect that LogisticRegression now supports multiclass. Also adds new examples to show multiclass training.

## How was this patch tested?
Ran locally using spark-submit, run-example, and copy/paste from user guide into shells. Generated docs and verified correct output.

Author: sethah <seth.hendrickson16@gmail.com>

Closes #15349 from sethah/SPARK-17239.