[SPARK-34399][DOCS][FOLLOWUP] Add docs for the new metrics of task/job commit time
[SPARK-34399][DOCS][FOLLOWUP] Add docs for the new metrics of task/job commit time

### What changes were proposed in this pull request?

This is follow-up of https://github.com/apache/spark/pull/31522.
It adds docs for the new metrics of task/job commit time

### Why are the changes needed?

So that users can understand the metrics better and know that the new metrics are only for file table writes.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Build docs and preview:
![image](https://user-images.githubusercontent.com/1097932/127198210-2ab201d3-5fca-4065-ace6-0b930390380f.png)

Closes #33542 from gengliangwang/addDocForMetrics.

Authored-by: Gengliang Wang <789196f1837e809005c9b5b207ae21d5a78ca970@apache.org>
Signed-off-by: Wenchen Fan <563f5923eae9a92c8a3f34a26b67fa348a303ed1@databricks.com>
(cherry picked from commit c9a7ff3f36838fad5b62fa5d9be020aa465e4193)
Signed-off-by: Wenchen Fan <563f5923eae9a92c8a3f34a26b67fa348a303ed1@databricks.com>

milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark