[DOC] SearchBackpressure stats needs additional details

[kaushalmahi12]

What do you want to do?
Currently the description of top level SearchBackpressure stats for SearchTask and SearchShardTask is not descriptive enough and incorrectly reflected on doc page.

What is missing ?
There is nothing explaining the differentiation and relation between resource tracker stats and top level stats in the response field. The sample response also seem to capture incorrect numbers.

the top level stats description should look something like the following

• search_backpressure.search_task - **This structure holds the resource tracker level statistics and the summary of cancellation stats for all of the resource trackers. Summary here means, search_backpressure.search_task.cancellation_stats value sums up cancellation_count from each of the resource tracker stats.
• search_backpressure.search_task.cancellation_stats - **This structure holds the total cancellations trigerred across all resource trackers and how many of those did not due to rate limit threshold. Mathmatically it is
  search_backpressure.search_task.cancellation_stats.cancellation_count + search_backpressure.search_task.cancellation_stats.cancellation_limit_reached_count = ∑ resource_tracker.cancellation_count

The same logic applies to shard_task stats component as well.

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*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

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