[DOC] Update _tasks API documentation with concurrency related changes

[jed326]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
As a part of introducing concurrent segment search we have also updated the _tasks API to return thread information and more detailed summary stats in cases with concurrency.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/OpenSearch#7673
opensearch-project/OpenSearch#7425

[hdhalter]

Are we targeting release 2.9/3.0 for this update?

[andrross]

@hdhalter We are targeting 2.9