[DOC] Update documentation for cluster settings to include Async shard batch fetch settings.

[Pranshu-S]

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.
Recent optimisations were made in the Async Shard Fetch flow to now fetch in batches of Shards on a node level basis instead of being executed per shard per node. The documentation changes required to be updated include settings to -

1. Enable Async Batch Fetch on existing shard allocator
2. Set Batch sizes for the Gateway allocators

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/OpenSearch#5098
opensearch-project/OpenSearch#8746

[hdhalter]

@Pranshu-S - Is this for 2.14 or should we backport it to 2.13?