[Messenger] Show message & handler(s) class description in debug:messen…

[fabpot]

Q	A
Feature PR	symfony/symfony#35978
PR author(s)	@ogizanagi
Merged in	5.1-dev

[javiereguiluz]

I don't understand this new feature well, but the only place where we show the output of debug:messenger is at the end of this article: https://github.com/symfony/symfony-docs/blob/master/messenger/multiple_buses.rst

@ogizanagi should we update that article? Thanks.

[ogizanagi]

Actually I didn't think this feature was worth a documentation update, but since we're showing its output, why not.

This feature adds in the output the messages & handlers descriptions from their docblock, allowing to document the use-cases they cover in the application. In a similar fashion as the debug:autowiring command showing autowirable interfaces description.