doc: document diverging MessagePort.onmessage handling

Fixes: https://github.com/nodejs/node/issues/26463

PR-URL: https://github.com/nodejs/node/pull/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>