Improve documentation for event registry and handling of failed events

[haagenhasle]

I asked Oliver a question about the event publication and externalization of events on Bluesky, and he asked me to create a ticket asking for the docs to be more precise about this. So here goes. :)

After reading the documentation, I was not sure what would happen if the broker I wanted to externalize an event to was down, if the message would be lost or if the event was backed by the registry in the same way as when events are handled internally. I have now learned that event externalization is just another listener and will be treated as such, but this could be clearer in the documentation.

Another thing that might be clearer is if it is the event itself that is stored in the registry or if there are individual entries in the registry for each event listener. If I have two event listeners for the same event and one of them fails, will only the one that failed be re-triggered or will both listeners be re-triggered?

Thanks for a great addition to the Spring ecosystem!