Comment by themulticaster

In contrast to the current sibling replies, I think this is a very fitting categorization of documentation. Off the top of my head, I can think of several examples where one type of documentation is excellent but others are very lacking, for example:

* Rust Library Documentation: Most libraries have complete and up-to-date reference documentation, but are lacking even basic introductions (tutorials/guides) on how to use the library. This is totally just my personal experience so maybe I've been looking at the wrong crates, but with most of the crates I spend several minutes looking trough all the modules in order to find that all the juicy functions are hidden in the Connection struct, or something similar.

* Linux Kernel Documentation: The Linux kernel has excellent in-depth explanations on several high-level concepts, but on the other hand a little more systematic reference documentation on the supporting library code would help a lot.

* While I can't think of a good example right now, a lot of projects have a few basic getting-started tutorials but don't explain advanced concepts or high-level design at all, leaving you to wade through the sources yourself in order to understand how to actually use them.