Document an overview of action names and how they're used in the transport layer

[DaveCTurner]

As noted at #100162 (comment) we don't really have any great overview docs about how transport actions are registered and used.

The Javadocs for org.elasticsearch.transport.TransportHandshaker describe the message format of transport handshakes which are pretty similar to regular transport message but we should have something similar about regular transport messages too. The key point there is that the action name is included (in plain ASCII) in the header (see org.elasticsearch.transport.Header) and we use that string to pick the handler from the map of registered handlers:

elasticsearch/server/src/main/java/org/elasticsearch/transport/InboundHandler.java

Lines 229 to 231 in 6caf9c1

	final String action = header.getActionName();
	final long requestId = header.getRequestId();
	final RequestHandlerRegistry<T> reg = requestHandlers.getHandler(action);

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DiannaHohensee]

IIUC, each TransportAction has a actionName, like this, and these paths/names/commands are registered someplace via the TransportService (?). Then when a node receives a transport layer request, either from another cluster node or internally, the actionName is looked up in some map that will cause the appropriate TransportAction to be invoked? I'm guessing the Handler naming convention is in reference to Netty and ChannelHandler extensions/implementations?

Not necessary to answer this now, but maybe this will give whomever picks this task up an additional clue as to what would be helpful to document.

[DaveCTurner]

these paths/names/commands are registered someplace

Specifically they're held in org.elasticsearch.transport.Transport.RequestHandlers#requestHandlers