Another thought I've had in the back of my mind is that "Using Moodle" is in fact the most comprehensive database of documentation that we have
A very true statement. There is an article (transcribed presentation) called Splitting Books Open: Trends in Traditional and Online Technical Documentation on O'Reilly's website exploring this issue. Though often written from the point of view of a publisher (and becoming something of an advert in the middle) I think it picks up on a lot of the trends that have made Using Moodle such an excellent resource.
You could probably distil some alternative 'documentation' guidelines from it, such as asking regular contributors to consider the wider audience (and Google) when answering tech questions.