While I have not looked at the documentation in the marketplace, I think that would be additional third party documentation.
I used to print copies of the documentation that is found on the downloads page and it has become well read and always on my desk, however it's value has declined as DotNetNuke has evolved at a fast rate and now many sections are more confusing as it is out of sync with current versions.
From time to time a few project leads will make the effort to blog about recent changes, and for me that is normally enough to review and test to get upto speed, however this is also rare.
A case in point is the latest NewsFeeds module (4.0.0) which behaves quite differently from previous versions, it made it's way into the core release of DNN and without some documentation regarding these changes, it had to be removed from our clients upgrades because it had undesirable results.
I think the original documentation was excellent, however to achieve the same quality of documentation on a regular basis would be quite a task, I would settle for just a desent blog about the changes until the documentation catches up.
When a new version of modules are released the first place I go to is the projects pages, however many times there is no real information regarding feature changes except for perhaps some bullet points.
Over recent months I have noticed a change from the core team and the organisation, I can't quite put my finger on it, but certainly getting information from core members appears diifficult, I recall when Joe Brinkman announced that 4.9.0 R1 was released to people who have access to RC's that I posted two messages asking how to get it or what qualifies a person to join the group, sadly he just chose to ignore my posts. With the lack of good information it makes it difficult to respond to clients and users of DotNetNuke in a timely manner and as a result it can devalue the project. We walk with caution with DotNetNuke since it bagan to saparate the community into various groups based on their ability to contibute financially to gain access to timely information. In some cases this has made us look at alternative solutions rather than exclusively putting DotNetNuke forward.