[DOC]Documentation - Enable merge on refresh and merge on commit in Opensearch 2.1.0+ (#5804)

* [DOC]Documentation - Enable merge on refresh and merge on commit in Opensearch 2.1.0+

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>

* Update _install-and-configure/configuring-opensearch/index-settings.md

Co-authored-by: Melissa Vagi <vagimeli@amazon.com>
Signed-off-by: Andriy Redko <drreta@gmail.com>

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Signed-off-by: Andriy Redko <andriy.redko@aiven.io>
Signed-off-by: Andriy Redko <drreta@gmail.com>
Co-authored-by: Melissa Vagi <vagimeli@amazon.com>

_install-and-configure/configuring-opensearch/index-settings.md

@@ -94,6 +94,13 @@ OpenSearch supports the following static index-level index settings:
 
 - `index.merge.policy` (String): This setting controls the merge policy for the Lucene segments. The available options are `tiered` and `log_byte_size`. The default is `tiered`, but for time-series data, such as log events, we recommend that you use the `log_byte_size` merge policy, which can improve query performance when conducting range queries on the `@timestamp` field. We recommend that you not change the merge policy of an existing index. Instead, configure this setting when creating a new index.
 
+- `index.merge_on_flush.enabled` (Boolean): This setting controls Apache Lucene's merge-on-refresh feature that aims to reduce the number of segments by performing merges _on refresh_ (or in terms of OpenSearch, _on flush_). Default is `true`.
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+- `index.merge_on_flush.max_full_flush_merge_wait_time` (Time unit): This setting sets the amount of time to wait for merges when `index.merge_on_flush.enabled` is enabled. Default is `10s`.
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+- `index.merge_on_flush.policy` (default | merge-on-flush): This setting controls which merge policy should be used when `index.merge_on_flush.enabled` is enabled. Default is `default`.
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 ### Updating a static index setting
 
 You can update a static index setting only on a closed index. The following example demonstrates updating the index codec setting.