DX: Documentation for incoming message workflow

First off, thanks to the maintainers for a great module. Building "new" stuff in D8 is a lot easier than in D7, but it's still nice to not start from scratch.

I am wondering about the "correct" way to handle incoming messages, vis-a-vis integrating with the framework instead of baking my own.

At the moment it appears there have been a few iterations of this, finally landing on calling the incoming() method on the default SMS provider service. This method takes an SmsMessage object, runs the message through the gateway's plugin incoming() method, and also fires pre- and post- events.

The question is, what's the standard method for first ingesting the incoming message and then preparing the SMS message for further processing? Is this expected to be provided by a callback in the plugin's module? And is it expected that the initial receipt of the SMS would implement verification of the incoming webhook signature, etc?

It would be great to include a summary of incoming message workflow in the docs. I can contribute this, I just need to know what the intended workflow is, first. :-)