With applications or libraries its easy to lose sight of the forest for the trees. It’s easy to get lost in the details and forget the broad strokes of what the applicaiton or library or module is trying to accomplish. If that’s true for the authors of said application or library, imagine how a user feels.
That’s why my favorite type of documentation is a glossary. This is just a list of terms related to the library as well as a brief description of how they all fit together.
While the rest of the documentation can delve deep, a glossary can give a user a sense of how things fit together at a glance.
Can’t write a concice glossary of a library or module? Chances are the design could use some work.