Document new experimental HTTP transport based on Netty 4 and Project Reactor (#6265)

* Document new experimental HTTP transport based on Netty 4 and Project Reactor

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

* Address code review comments

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

* Address code review comments

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

---------

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

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

@@ -48,3 +48,12 @@ OpenSearch supports the following advanced network settings for transport commun
 - `transport.bind_host` (Static, list): Specifies an address or addresses to which an OpenSearch node binds to listen for incoming transport connections. 
 
 - `transport.publish_host` (Static, list): Specifies an address or addresses that an OpenSearch node publishes to other nodes for transport communication.
+
+## Selecting the transport
+
+The default OpenSearch transport is provided by the `transport-netty4` module and uses the [Netty 4](https://netty.io/) engine for both internal TCP-based communication between nodes in the cluster and external HTTP-based communication with clients. This communication is fully asynchronous and non-blocking. However, there are other transport plugins available that can be used interchangeably:
+
+Plugin | Description
+:---------- | :--------
+`transport-nio`    | The OpenSearch transport based on Java NIO. <br> Installation: `./bin/opensearch-plugin install transport-nio` <br> Configuration (using `opensearch.yml`): <br> `transport.type: nio-transport` <br> `http.type: nio-http-transport`
+`transport-reactor-netty4`    | The OpenSearch HTTP transport based on [Project Reactor](https://github.com/reactor/reactor-netty) and Netty 4 (**experimental**) <br> Installation: `./bin/opensearch-plugin install transport-reactor-netty4` <br> Configuration (using `opensearch.yml`): <br> `http.type: reactor-netty4`