Docs disambiguate reindex's requests_per_second (#26185)

Reindex's docs were somewhere between unclear and
inaccurate around `requests_per_second`. This makes
them much more clear and accurate.

Author: berglh

docs/reference/docs/reindex.asciidoc

@@ -534,14 +534,21 @@ shards to become available. Both work exactly how they work in the
 <<docs-bulk,Bulk API>>.
 
 `requests_per_second` can be set to any positive decimal number (`1.4`, `6`,
-`1000`, etc) and throttles the number of requests per second that the reindex
-issues or it can be set to `-1` to disabled throttling. The throttling is done
-waiting between bulk batches so that it can manipulate the scroll timeout. The
-wait time is the difference between the time it took the batch to complete and
-the time `requests_per_second * requests_in_the_batch`. Since the batch isn't
-broken into multiple bulk requests large batch sizes will cause Elasticsearch
-to create many requests and then wait for a while before starting the next set.
-This is "bursty" instead of "smooth". The default is `-1`.
+`1000`, etc) and throttles the number of batches that the reindex issues by 
+padding each batch with a wait time. The throttling can be disabled by 
+setting `requests_per_second` to `-1`.
+
+The throttling is done waiting between bulk batches so that it can manipulate the 
+scroll timeout. The wait time is the difference between the request scroll search
+size divided by the `requests_per_second` and the `batch_write_time`. By default 
+the scroll batch size is `1000`, so if the `requests_per_second` is set to `500`:
+
+`target_total_time` = `1000` / `500 per second` = `2 seconds` +
+`wait_time` = `target_total_time` - `batch_write_time` = `2 seconds` - `.5 seconds` = `1.5 seconds`
+
+Since the batch isn't broken into multiple bulk requests large batch sizes will 
+cause Elasticsearch to create many requests and then wait for a while before 
+starting the next set. This is "bursty" instead of "smooth". The default is `-1`.
 
 [float]
 [[docs-reindex-response-body]]