Even before 1990. Over forty years ago when I had just begun in software, we started by writing the user manual. Well, actually, we didn't. We described what the customer would want to achieve in a very short document (probably just a bit longer than press release length), then wrote the user manual. Of course, the actual user manual would describe what we eventually built, and so would differ from the original one, but the point remains that we built things for the customer.