[Question] Why I need -b parameter to receiver

Sorry for question in issues but I miss one thing from the documentation that prevents me from finishing integration.

I have configuration:
```
messenger:
buses:
mailSenderBus:
singleHandlerPerMessage: true
transports:
postgres: 'doctrine://default'
routing:
SendMailRequest: postgres

services:
mailSenderBus: MailSenderBus(@messenger.mailSenderBus.bus)
mailSenderHandler:
class: MailSenderHandler
doctrineTransportFactory:
class: Symfony\Component\Messenger\Bridge\Doctrine\Transport\DoctrineTransportFactory
tags: [messenger.transportFactory]
```

Messages are enqueued OK, yet when I run
```
bin/console messenger:consume -vvv postgres
```

The messages are not handed... I have to run this to make it work
```
bin/console messenger:consume -vvv postgres -b mailSenderBus
```

Why? Is that desired? The expected behaviour is that the first command should process all the messages, instead it gets to the envelope and silently discards it... which is 🤯

Thanks for clarification

Reproduce the configuration and compare `bin/console messenger:consume -vvv postgres` with the same command using `-b mailSenderBus`. Start by tracing how the receiver selects or handles the bus and why the envelope is discarded without the option. Done means the documented command behavior is clarified or corrected, with coverage for the shown configuration and both commands.

Written by the indexing model from the issue text.