[DOCS][MINOR] Clarify Where AccumulatorV2s are Displayed

## What changes were proposed in this pull request?

This PR clarifies where accumulators will be displayed.

## How was this patch tested?

No testing.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Bill Chambers <bill@databricks.com>
Author: anabranch <wac.chambers@gmail.com>
Author: Bill Chambers <wchambers@ischool.berkeley.edu>

Closes #16180 from anabranch/improve-acc-docs.