[DOC] k-NN plugin: add reference to hybrid mmap extensions setting under performance tunning

[martin-gaievski]

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• Request a change to existing documentation
• Add new documentation
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Tell us about your request.
We need to add description of a performance tuning of the k-NN plugin. Possible place is a new section under https://opensearch.org/docs/latest/search-plugins/knn/performance-tuning/#search-performance-tuning.

Content can be something along these lines:

Use mmap file I/O

For approximate k-NN search that is based on Lucene engine there is no special cache layer that speeds up read/write operations, plugin is relying on existing mechanism in OpenSearch core. By default in versions 2.4 and below such operations were based on Java NIO, that can be slow depending on the Lucene version and number of segments per shard.
Starting from version 2.5 k-NN enables mmap file I/O by default when store type is hybridfs (this is default store type in OpenSearch). This enables fast file I/O operations and improves overall performance of both data ingestion and search. Two file extensions specific to vector values are marked for mmap, ".vec" and ".vem", please check Lucene documentation for details of each extensions.

MMap uses system file cache rather than memory allocated for Java heap, no additional allocation is required. In case it's required to change default list of extensions set by the plugin, please update index.store.hybrid.mmap.extensions setting at the cluster level using Cluster API. Please note, this is an expert level setting, index needs to be closed before update, and the re-open.

What other resources are available?
Cx request that triggered investigation and change: opensearch-project/k-NN#637