[DOC] Document cluster.default_number_of_replicas and update index.number_of_replicas

[chishui]

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• 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.
cluster.default_number_of_replicas is an existing configuration but never documented. index.number_of_replicas uses cluster.default_number_of_replicas as default value which is also not documented correctly.

As two index.number_of_replicas related bugs were fixed, we should have the correct documentation for the use of these two configurations.

opensearch-project/OpenSearch#14810
opensearch-project/OpenSearch#14783

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
OS 2.18.

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