By far, the worst docs I've read was for a library I used to use for almost every project. I didn't really have to look at the docs because I knew the ins and outs of it. Time went by and I stopped using the library. I came back to a project that used that library, and I had the hardest time figuring out what was going on.

It was a library I wrote :/

I got much better at documentation after that. I started doing DDD (Document Driven Development) because many developer's first experiences with libraries are with the documentation. It allowed me to interact with my library before I even started development.