Glibc change exposing bugs

Yes, they are the most useful documentation we have, especially for things that do not have glibc wrappers. But even for the kernel they are descriptive, and for things for which the glibc wrappers are the primary implementation (like readdir()) or for which there is no kernel component, the manpages are completely after-the-fact. (As far as I can tell the glibc project no longer bothers to document anything at all. There are lots of utterly undocumented things in glibc's allegedly public interface.)