You would have to basically create an endowment to fund ongoing documentation development.
Agreed that continued funding would be necessary to the extent that renewed documentation is needed. Whether an endowment or repeat crowdfunding is the best mechanism for doing so would probably vary from project to project. Perhaps you make the endowment approach a big stretch goal; like "$18,000 base funding for a one-time project, $250,000 or more creates an endowment with three annual $20,000 update projects until the endowment (invested in broad-based low risk equity funds, 50% domestic, 25% foreign first world, 25% foreign developing nations) is depleted" -- but I digress, you get the idea.
A) the interfaces are bad enough that documentation is even necessary in the first place
As you imply, I find that good documentation often exposes opportunities for improvement in the interface. That could become a channel for providing recommendations to the core development team, or could become the seed for a third-party development effort. Things which have value can get built, either because the developers and their sponsors want them, or through crowdfunding, or through some other motivating mechanism.
In short; you've raised an opportunity to create additional value, not a threat to succeeding in the base objective.
B) documentation is boring, unrewarding and time consuming to do well so nobody wants to bother.
That is a restatement of the original premise for which we are attempting to find potential solutions. I think I am missing your intent in raising it anew as a bullet point.