[DOC] Document support for index level max_slice_count setting and search.concurrent_segment_search.mode setting

[Gankris96]

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.

Adding to the changes mentioned in opensearch-project/OpenSearch#15268, we will also need to document the availability of the setting at index level.

As part of the related to this issue - opensearch-project/OpenSearch#15259
In PR: opensearch-project/OpenSearch#15363

we will be introducing a new index and cluster setting

Cluster:
search.concurrent_segment_search.mode

Index:
index.search.concurrent_segment_search.mode

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

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

opensearch-project/OpenSearch#15336 (comment) Something along these lines to show how the setting can be updated and how it would work in tandem with cluster level setting.

[Naarcha-AWS]

@Gankris96: To make sure we have everything we need, please submit a PR with the index-level slice settings after the development PR has been merged and successfully backported.