Maybe premature documentation isn’t the root of all evil?
I get that technical documentation really should only be written during the final development iteration, and that anything sooner is a very effective way to wast time and trap yourself in local maxima.
But surely it’s never too early to be thinking about the usage experience? Writing that down can help solidify the vision of the actual product, the thing people use.
When you first conceive of it, you probably know why you want it, so you know what problem this is supposed to be solving. That’s your purest perspective, right there. That’s the perspective that needs to be writing the user-side spec.