Finish chewing-utf8-util.h document.

I'm going to start a `cleanup` program. It has two main purposes:

1. Document all files.
2. Make the coding style across all files consistent.

For 1, I'll emphasise on Doxygen more. I think it's easier to browse functions on html.

So I'll start with some basic utility program like the file in this commit, and I hope I can get better understanding of chewing IM by doing this.