My answer, originally published on Quora.
Q: Is a product manager responsible for product documentation?
Product managers are typically responsible for the "whole product" -- which means the product itself, documentation, packaging, pricing, positioning and sometimes also the promotion of those products.
But being responsible for documentation does not mean that product managers actually WRITE the documentation. Usually a professional writer does that. Just as professional developers usually write the code.
But note that I said usually. I have worked with a few early stage startups without the resources to hire a writer, so the product manager wrote it. But in those same startups, the CEO often cleaned the bathrooms and stocked the fridge. In all cases, it was a temporary thing.