I remember finding a good number of system-call manpages discussions in LKML. Otherwise, where should we find documentation for things like futexes, netlink sockets, and the rest?
For good or bad, these manpages are the 'most primary' sources available for such topics, only beside the code.
But unfortunately these man-pages do not always exist. I once had to carefully study the bluez userspace code to know how to best interface with the kernel Bluetooth API (undocumented AF_BLUETOOTH sockets, undocumented netlink interfaces, etc).