Message Events Process is confusing for users

[npepinpe]

Copied over from the Zeebe repo: camunda/camunda#4422

Description

The process shown at Message Events is a bit confusing, since it seems the service task is directly completed and we wait until we receive a message which tells us that the money was collected.

We should try to re-model this process.

Related Zeebe Forum post

https://forum.zeebe.io/t/is-the-zeebe-task-blocking-or-synchronous/1183

[akeller]

@menski can you move this forward please?