[Messenger] Document "own Receiver"

[xfifix]

it's possible someone explain "own receiver" with a case.
The doc just show the class who implement ReceiverInterface and not how to use it...
thank a lot

[xfifix]

@Nyholm 🙏

[Nyholm]

Thank you for the ping.

Sure, I can write up something. @xfifix could you help me review the PR and ask 1000 questions about things that are not clear and needs to be explained better?

[xfifix]

@Nyholm I understand that you don't have the time.

[Nyholm]

Im saying that Im happy to do it. But I need help reviewing it and I asked if you could help me review it later?

[Nyholm]

@xfifix Could you please review: #12340

[xfifix]

@Nyholm Thank you for taking the time for this PR.