I don't know if this is a hard rule for Symfony docs, but the framework integration usually is documented in the Guides section or a dedicated page, not in the Component documentation itself.
Therefore, you can already find a doc entry accessible from the "Guides" menu showing config samples & multiple buses wiring:https://symfony.com/doc/current/messenger.html#multiple-buses.
In any case, this PR would be a nice addition to the existing section. 👍

Please describe how to register command/event/query handlers and how to seggregate them between buses. Nobody wants handle received message from from event bus through middlewares for command bus.

@Koc's comment relates to#10477. Indeed this PR might be a good place for documenting it too, but not mandatory.

Appart from the mention of a returned value in a query bus, which is not possible anymore, I think this PR is ready.

I've continued your work in#10689 , adding some more things (including@Koc 's comment).

Thank you@magnusnordlander and@ogizanagi

Inhttps://symfony.com/doc/master/messenger/multiple_buses.html there is no example for a routing configuration for multiple buses. It would be nice if there was a typical example configuration, something like:
Route all messages of messenger.bus.events to a amqp transport, handle all messages of messenger.bus.commands synchronously.