## What changes were proposed in this pull request?

Similar to SPARK-18401, as a classification algorithm, logistic regression should support output original label instead of supporting index label.

In this PR, original label output is supported and test cases are modified and added. Document is also modified.

## How was this patch tested?

Unit tests.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #15910 from wangmiao1981/audit.