Document cluster.default_number_of_replicas and update index.number_o…

…f_replicas (opensearch-project#8526)

* Document cluster.default_number_of_replicas and update index.number_of_replicas

Signed-off-by: Liyun Xiu <xiliyun@amazon.com>

* Apply suggestions from code review

Signed-off-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>

---------

Signed-off-by: Liyun Xiu <xiliyun@amazon.com>
Signed-off-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>
Co-authored-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>

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

@@ -79,6 +79,8 @@ OpenSearch supports the following dynamic cluster-level index settings:
 
 - `cluster.snapshot.shard.path.prefix` (String): Controls the fixed path prefix for snapshot shard-level blobs. This setting only applies when the repository `shard_path_type` setting is either `HASHED_PREFIX` or `HASHED_INFIX`. Default is an empty string, `""`.
 
+- `cluster.default_number_of_replicas` (Integer): Controls the default number of replicas for indexes in the cluster. The index-level `index.number_of_replicas` setting defaults to this value if not configured. Default is `1`.
+
 ## Index-level index settings
 
 You can specify index settings at index creation. There are two types of index settings:
@@ -185,7 +187,7 @@ For more information about updating settings, including supported query paramete
 
 OpenSearch supports the following dynamic index-level index settings:
 
-- `index.number_of_replicas` (Integer): The number of replica shards each primary shard should have. For example, if you have 4 primary shards and set `index.number_of_replicas` to 3, the index has 12 replica shards. Default is 1.
+- `index.number_of_replicas` (Integer): The number of replica shards each primary shard should have. For example, if you have 4 primary shards and set `index.number_of_replicas` to 3, the index has 12 replica shards. If not set, defaults to `cluster.default_number_of_replicas` (which is `1` by default).
 
 - `index.auto_expand_replicas` (String): Whether the cluster should automatically add replica shards based on the number of data nodes. Specify a lower bound and upper limit (for example, 0--9) or `all` for the upper limit. For example, if you have 5 data nodes and set `index.auto_expand_replicas` to 0--3, then the cluster does not automatically add another replica shard. However, if you set this value to `0-all` and add 2 more nodes for a total of 7, the cluster will expand to now have 6 replica shards. Default is disabled.