[DOC] Index setting for Search Replica Auto expand

[vinaykpud]

What do you want to do?

Similar to index.auto_expand_replicas settings for the replicas as given in the documentation, recently we have added a new setting index.auto_expand_search_replicas for search replicas. This was added as part of PR: opensearch-project/OpenSearch#17741

• 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.
Add documentation for the setting index.auto_expand_search_replicas

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

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

Reader Writer separation meta and issues: opensearch-project/OpenSearch#15306
PR: opensearch-project/OpenSearch#17741

[opensearch-ci-bot]

Hi @Naarcha-AWS
,

As part of the entrance criteria, all the documentation pull requests need to be drafted and in technical review.

Since there is no pull request linked to this issue, please take one of the following actions:

• Create the pull request and link it to this issue.
  
• If you already have a pull request created, please link it to this issue.
  
• If this feature is not targeted for the currently labeled release version, please update the issue with the correct release version.
  

Please note: Missing documentation can block the release and cause delays in the overall process.

Thank you!

[vinaykpud]

Here is the documentation draft:

index.auto_expand_search_replicas (String):
Specifies whether the cluster should automatically adjust the number of search replica shards based on the number of available search nodes. The value should be provided as a range, with a lower and upper bound — for example, 0-3 or 0-all.

• If set to 0-3 and the cluster has 5 data nodes, the index can have up to 3 search replicas. Even if more nodes are added, no additional replicas will be created beyond the specified upper bound.
• If set to 0-all and you increase the number of data nodes from 5 to 7, the cluster automatically expands the number of search replicas to match — in this case, 7.

Default: Disabled.