[SPARK-22141][FOLLOWUP][SQL] Add comments for the order of batches

## What changes were proposed in this pull request?
Add comments for specifying the position of  batch "Check Cartesian Products", as rxin suggested in https://github.com/apache/spark/pull/19362 .

## How was this patch tested?
Unit test

Author: Wang Gengliang <ltnwgl@gmail.com>

Closes #19379 from gengliangwang/SPARK-22141-followup.