Part of the problem is that the people working on the documentation aren't the same people making the code changes (for the most part). Then there's also the fact that a lot of documentation is somewhat duplicated, and it requires that a member of the documentation clan see a change and remember which documentation it will affect, and then change it all. So far as I know there is no general outline of all documentation--something I'd especially like to see written (but note that I've been inactive for the last couple of months due to moving).

Therefore, the best thing to do when you note some documentation which is out of date is to mention it to the site documentation clan. If you find that you're doing this a lot, you could request to join sdc.