Besides those few found in the glibc-doc package (which probably should
just be taken over, given that that package really consists of only a
number of pthread manpages and… imperial tons of changelogs?!), there are
manpages genuinely missing, such as pthread_barrier_init, pthread_barrier_wait
and friends.

Funnily enough, they apparently used to exist (in another package) in
stretch…

Could be useful combing through libc and looking for more undocumented APIs
where such documentation is obtainable elsewhere…