Years ago I worked on the documentation for one of those big game engines. Previously as a "user" I suffered a lot with their physics system, and there was a big gap about its performance and best practices. After a long research in the source code and emails back and forth in the company I was able to fill those holes. My motivation came completely from within me, because absolutely no one in the company seemed interested on fixing the docs. It's the least important thing in most of these big companies.