[Backport 2.3] Adds max_shard_size parameter to shrink API

_api-reference/index-apis/shrink-index.md

@@ -57,10 +57,21 @@ timeout | Time | How long to wait for the request to return a response. Default
 You can use the request body to configure some index settings for the target index. All fields are optional.
 
 Field | Type | Description
-alias | Object | Sets an alias for the target index. Can have the fields `filter`, `index_routing`, `is_hidden`, `is_write_index`, `routing`, and `search_routing`. See [Index Aliases]({{site.url}}{{site.baseurl}}/api-reference/alias/#request-body).
+:--- | :--- | :---
+alias | Object | Sets an alias for the target index. Can have the fields `filter`, `index_routing`, `is_hidden`, `is_write_index`, `routing`, or `search_routing`. See [Index Aliases]({{site.url}}{{site.baseurl}}/api-reference/alias/#request-body).
 settings | Object | Index settings you can apply to your target index. See [Index Settings]({{site.url}}{{site.baseurl}}/api-reference/index-apis/create-index/#index-settings).
+[max_shard_size](#the-max_shard_size-parameter) | Bytes | Specifies the maximum size of a primary shard in the target index. Because `max_shard_size` conflicts with the `index.number_of_shards` setting, you cannot set both of them at the same time. 
+
+### The `max_shard_size` parameter
+
+The `max_shard_size` parameter specifies the maximum size of a primary shard in the target index. OpenSearch uses `max_shard_size` and the total storage for all primary shards in the source index to calculate the number of primary shards and their size for the target index. 
+
+The primary shard count of the target index is the smallest factor of the source index's primary shard count for which the shard size does not exceed `max_shard_size`. For example, if the source index has 8 primary shards, they occupy a total of 400 GB of storage, and the `max_shard_size` is equal to 150 GB, OpenSearch calculates the number of primary shards in the target index using the following algorithm:
+
+1. Calculate the minimum number of primary shards as 400/150, rounded to the nearest whole integer. The minimum number of primary shards is 3.
+1. Calculate the number of primary shards as the smallest factor of 8 that is greater than 3. The number of primary shards is 4.
+
+The maximum number of primary shards for the target index is equal to the number of primary shards in the source index because the shrink operation is used to reduce the primary shard count. As an example, consider a source index with 5 primary shards that occupy a total of 600 GB of storage. If `max_shard_size` is 100 GB, the minimum number of primary shards is 600/100, which is 6. However, because the number of primary shards in the source index is smaller than 6, the number of primary shards in the target index is set to 5.
 
-{% comment %}
-The new parameter to be called max_shard_size. 
-max_primary_shard_size | Bytes | Sets the maximum size of a primary shard in the target index. For example, if this field is set to 100 GB, and the source index's primary shards total to 300 GB, then the target index has 3 primary shards of 100 GB each.
-{% endcomment %}
+The minimum number of primary shards for the target index is 1.
+{: .note}