Documentation that isn't in the contract doesn't get written, it gets promised, then deferred, then forgotten. Treat it as a deliverable with a definition of done, like any feature.
The bar is concrete: a README that gets a new developer running in an afternoon, an operations doc that covers deploy and restore, and a handover session where both are walked through.
When docs are a deliverable, they get scheduled, reviewed, and signed off. When they're a favor, they get skipped. The difference is a line in the scope.
