[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 <[email protected]>
Author: anabranch <[email protected]>
Author: Bill Chambers <[email protected]>

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

(cherry picked from commit 70ffff2)
Signed-off-by: Sean Owen <[email protected]>

Author: Bill Chambers

docs/programming-guide.md

@@ -1345,14 +1345,15 @@ therefore be efficiently supported in parallel. They can be used to implement co
 MapReduce) or sums. Spark natively supports accumulators of numeric types, and programmers
 can add support for new types.
 
-If accumulators are created with a name, they will be
-displayed in Spark's UI. This can be useful for understanding the progress of
-running stages (NOTE: this is not yet supported in Python).
+As a user, you can create named or unnamed accumulators. As seen in the image below, a named accumulator (in this instance `counter`) will display in the web UI for the stage that modifies that accumulator. Spark displays the value for each accumulator modified by a task in the "Tasks" table.
 
 <p style="text-align: center;">
   <img src="img/spark-webui-accumulators.png" title="Accumulators in the Spark UI" alt="Accumulators in the Spark UI" />
 </p>
 
+Tracking accumulators in the UI can be useful for understanding the progress of 
+running stages (NOTE: this is not yet supported in Python).
+
 <div class="codetabs">
 
 <div data-lang="scala"  markdown="1">