Add grpc.netty.executor_count setting to GRPC documentation (#10937)

* Add grpc.netty.executor_count setting to GRPC documentation

Signed-off-by: Karen Xu <karenxyr@gmail.com>

* Update _api-reference/grpc-apis/index.md

Co-authored-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>
Signed-off-by: Karen X <karenxyr@gmail.com>

---------

Signed-off-by: Karen Xu <karenxyr@gmail.com>
Signed-off-by: Karen X <karenxyr@gmail.com>
Co-authored-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>

Author: karenyrx

_api-reference/grpc-apis/index.md

@@ -74,6 +74,7 @@ OpenSearch supports the following advanced settings for gRPC communication. Thes
 | `grpc.bind_host` | A list of addresses to bind the gRPC server to. Can be distinct from publish hosts. | `["0.0.0.0", "::"]` | Value of `grpc.host` |
 | `grpc.publish_host` | A list of hostnames or IPs published to peers for client connections. | `["thisnode.example.com"]` | Value of `grpc.host` |
 | `grpc.netty.worker_count` | The number of Netty worker threads for the gRPC server. Controls concurrency and parallelism. | `2` | Number of processors |
+| `grpc.netty.executor_count` | The number of threads in the fork-join pool for processing gRPC service calls. Controls request processing parallelism. | `32` | `2 * number of processors` |
 | `grpc.netty.max_concurrent_connection_calls` | The maximum number of simultaneous in-flight requests allowed per client connection. | `200` | `100` |
 | `grpc.netty.max_connection_age` | The maximum age a connection can reach before being gracefully closed. Supports time units like `ms`, `s`, or `m`. See [Time units]({{site.url}}{{site.baseurl}}/api-reference/common-parameters/#time-units). | `500ms` | Not set (no limit) |
 | `grpc.netty.max_connection_idle` | The maximum duration for which a connection can be idle before being closed. Supports time units like `ms`, `s`, or `m`. See [Time units]({{site.url}}{{site.baseurl}}/api-reference/common-parameters/#time-units). | `2m` | Not set (no limit) |