If you are using GNU C (glibc), the functions (beyond the GNU extensions) follow the POSIX standard as far as their arguments, implementation, failure and return values. If you want to peek under the hood of static members, you'll have to look at the code.
Every push (that I can remember) to try and adopt something like Doxygen for glibc was rejected for the following reasons:
- Redundant, POSIX already documents almost everything thats exposed, as well as man and info pages.
- Too much work initially
- More work for maintainers
As far as the kernel goes, Linux does use a system very similar to Doxygen called Kerneldoc.