I think there can be different types of documentation (tutorials, reference, explanation, how-to guides). I came across this article a while back that goes over different categories and always re-read it when I know I'm going to have to write a bunch of documentation for something and always find it helpful to really think about the type that I need to write. In regards to the poster you replied to I think Oreilly books are in the explanation category of documentation instead of the traditional reference kind found online.
The article I'm referring to: https://www.divio.com/blog/documentation/