I'm not sure they've got the boundary between Reference and Explanation quite right. They seem to be thinking of the Explanation category as less than formal,

I'm not sure they've got the boundary between Reference and Explanation quite right. They seem to be thinking of the Explanation category as less than formal, and of the Reference category as organised around individual items (a list of methods, or command-line arguments, or configuration options). What a lot of projects miss is formal, complete-and-correct documentation of what the program does and doesn't promise to do, independent of any user-controllable knob.