[SPARK-21100][SQL][FOLLOWUP] cleanup code and add more comments for Dataset.summary

## What changes were proposed in this pull request?

Some code cleanup and adding comments to make the code more readable. Changed the way to generate result rows, to be more clear.

## How was this patch tested?

existing tests

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18570 from cloud-fan/summary.