doc: document diverging MessagePort.onmessage handling

Fixes: #26463

PR-URL: #26487
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: addaleax

doc/api/worker_threads.md

@@ -319,6 +319,9 @@ listeners are attached.
 
 This method exists for parity with the Web `MessagePort` API. In Node.js,
 it is only useful for ignoring messages when no event listener is present.
+Node.js also diverges in its handling of `.onmessage`. Setting it will
+automatically call `.start()`, but unsetting it will let messages queue up
+until a new handler is set or the port is discarded.
 
 ### port.unref()
 <!-- YAML