Judging from the comments here, it appears I have completely failed at properly getting across just how early this thing is in development. I've since updated my README with a status report.
As for documentation, that is an active WIP. I have recently pinned down a documentation style I like, and applied it to the newest APIs; all that's left to do really is to spread it to the older APIs as well, and write a tool to turn that documentation into HTML, because I have not found a documentation tool that operates how I want it to.
Oh, and one other thing I forgot to mention about documentation: the nature of this project requires me to balance what I can get behaving identically across platforms, what I can guarantee about behavior across platforms that must differ, and what I am forced to leave as "implementation-defined" with no further specification. If this was a non-native project like GTK+ and Qt, I would be the one making these decisions, but in this case I am not, so I must take a lot of factors into consideration when writing documentation. If you read through the docs I did have on package ui, you'll find a lot of TODOs about behavior.
As for documentation, that is an active WIP. I have recently pinned down a documentation style I like, and applied it to the newest APIs; all that's left to do really is to spread it to the older APIs as well, and write a tool to turn that documentation into HTML, because I have not found a documentation tool that operates how I want it to.