Documentation gives at least the functional, if not detailed, requirements for what the code is supposed to do (as well as not do). "Submit a pull request for documentation" is basically "please write down the set of requirements for what you think I might have coded up here and send them to me" which is an absurd way to do engineering, regardless of considerations of licensing and compensation structure.

It's okay to have that attitude about tutorials, and guides to internals, and other satellite docs, but not core documentation (reference manual or spec).