Bad documentation covers obvious things very well, and doesn't cover tricky parts at all. This is especially what you get if the writer of the documentation is

Bad documentation covers obvious things very well, and doesn't cover tricky parts at all. This is especially what you get if the writer of the documentation is paid by the amount of written text: there is no incentive to spend time investigating the hard parts, but a lot of incentive to explain the easy parts in too much detail.