nit: Do you want to do type annotation for our python scripts?

Having the function signature encoded here makes more sense for replacing libc-hdrgen, but isn't strictly required for documentation purposes (at the moment; perhaps we could generate something akin to very very minimal `man` pages, eventually).

Perhaps `defined` should be a list. For example, if something is specified by both C and POSIX, maybe it makes sense to refer to BOTH sections? The C standard can't be hyperlinked to AFAIK unfortunately, but POSIX can be IIRC. Maybe supporting hyperlinking would be nice.

Rather than have one long long running file, perhaps it would be better to rely on a convention where we have individual yaml files, one per header such as ctype.h.yml and fenv.h.yml (or ctype.yml fenv.yml). Then the header name is implied by the file name, and we can keep the individual files shorter and perhaps better organized?