Documentation is one of those things that once there's a lot of it, it's its own product.

First, you have to have someone constantly making sure it's up to date in the case of software. Really easy for it to go stale and just be wrong, and possibly actively harmful.

But then telling someone "RTFM" has a different quality if you're talking about 50 pages or 50,000 pages. Cool, I'll get back to you in a month.