There's certainly cases where the documentation basically doesn't exist, or it's essentially the same as the design documentation, and the strategy is basically

There's certainly cases where the documentation basically doesn't exist, or it's essentially the same as the design documentation, and the strategy is basically that if you're a big enough customer then you'll be getting some engineer time to make things work, and if you're a smaller customer you'll be told to pound sand anyway, it's not worth them fixing up their documentation to get your business. Generally the more complex and specialised the hardware the more it'll look like this, and it basically saves them on R&D because they have only a few applications to actually focus on.