[DOC] Async search missing docs on "index" query parameter

[andrross]

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.
The asynchronous search documentation does not detail how to invoke a search against a single index by using the index query parameter. See this slack thread: https://opensearch.slack.com/archives/C051D137M7G/p1711633954357529

This relates to all OpenSearch versions.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
https://opensearch.org/docs/latest/search-plugins/async/index/

[IanMenendez]

@andrross I will add this