[SPARK-17710][FOLLOW UP] Add comments to state why 'Utils.classForName' is not used

## What changes were proposed in this pull request?
Add comments.

## How was this patch tested?
Build passed.

Author: Weiqing Yang <yangweiqing001@gmail.com>

Closes #15776 from weiqingy/SPARK-17710.

(cherry picked from commit 8a9ca192)
Signed-off-by: Reynold Xin <rxin@databricks.com>