Style guide is here:

https://github.com/cfengine/documentation/blob/master/README.md

with examples on how functions are to be documented (not taking into account the macros yet).

I'm replacing the hard-coded list of arguments and their ranges with the macro right now in the functions, check my last commit for some ideas on how to document arguments: https://github.com/cfengine/documentation/commit/e913e6fc0e5568d0c003e5e54354871b4b5f5779

I'm not sure if that works here, since you want to document each option value, which is quite verbose. So in that case, not using the macro and instead documenting the arguments explicitly might be the best thing to do. I have to do that for https://github.com/cfengine/documentation/blob/master/reference/functions/parseintrealstringarray.markdown for other reasons, so just follow the style that's used elsewhere.