When you teach programming, you teach reading, writing and logic as well.
Apart from that, the question is not whether the documentation is good or bad, but whether it's better or worse than the one a non-programmer would write.
Finally, even apart from writing skills, programming teaches you how to comprehend what you're reading (docs), and how to solve problems.
Nobody teaches basic English at the college level unless they're being paid specifically to bring remedial students up to speed (and before you ask: yes, that includes freshman English class). I've taught a few "writing intensive" lab courses, but that basically meant that we forced the students to write a lot, and that we were allowed to brutalize them for crappy writing (roughly 85% of the students). We spent no time teaching them how to write, nor could we: we barely had time to teach the subject at hand.