Personal experience seems to indicate that the best Designs do not need Documentation to be usable, but the worst Designs need pages and pages of Documentation. The heuristic is that the quality of a Design is inversely proportional to the length of the user Documentation.
The problem, of course, is that there’s no guarantee that a poorly-designed product has enough compensatory Documentation. Likewise, someone may have just written lots of Documentation for something that didn’t really need it.
Taken on its own, it’s not perfect, but if you are looking at adopting a product, process, or other technology, you may want to choose the one that, all else being equal, has the slimmest manual.