RED.notify
is documented: RED.notify : Node-RED
I don't have a strong opinion on whether using tray or dialog is better.
RED.tray
is available to use, will look at adding docs for it.
There are a few examples in the editor src you could crib from for now: node-red/packages/node_modules/@node-red/editor-client/src/js/ui at master · node-red/node-red · GitHub