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);