[SPARK-19659][CORE][FOLLOW-UP] Fetch big blocks to disk when shuffle-read

## What changes were proposed in this pull request?

This PR includes some minor improvement for the comments and tests in https://github.com/apache/spark/pull/16989

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18117 from cloud-fan/follow.