Save useful info about question walkthrough tests from the comment in MDL-77327

[timhunt]

What is the Moodle feature that needs documenting?

Well, the whole of the core_question system needs documenting. But, once that is done, we should capture (and probalby expand) the explanation I wrote in the tracker comment.

Is this documentation specific to a Moodle version?

No

Are you able to provide a patch for this?

Yes - I will create one