CLDOC is a document generator for C and C++ projects to create documentation automatically out of comments as well as creating XML-based descriptions of the API. CLDOC is a new project but already sounds promising and is being powered by LLVM's Clang C/C++ front-end for its parser.
More details on CLDOC for Clang-based document generation can be found in this blog post
by Jesse van den Kieboom. The code to CLDOC is hosted on GitHub