Many people have no problem using libui. There are even many bindings available that people use some of which have documentation. I was able to start using libui instantly, but mostly because I already have experience using other GUI libraries.

People that know C should be able to read and understand the interface file at:

https://github.com/andlabs/libui/blob/master/ui.h

The function / variable names are informative enough. There are examples which should get you started at:

https://github.com/andlabs/libui/tree/master/examples

Even if they would add comments to the interface files (that may be used to generate docs), it will not add much value if it will become as follows:

  // Set window title. First argument should be the window and the second the title as a string
  uiWindowSetTitle(uiWindow *w, const char *title);

What I think is more important is adding tests etc. Tests can then also serve as a kind of documentation.