LibPthread: Add POSIX spec links for pthread_once API

Author: https://github.com/bgianfo
Commit: https://github.com/SerenityOS/serenity/commit/29716b9624b
Pull-request: https://github.com/SerenityOS/serenity/pull/11691
Reviewed-by: https://github.com/Quaker762 ✅

Userland/Libraries/LibPthread/pthread_once.cpp

@@ -17,6 +17,7 @@ enum State : i32 {
     PERFORMING_WITH_WAITERS,
 };
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html
 int pthread_once(pthread_once_t* self, void (*callback)(void))
 {
     auto& state = reinterpret_cast<Atomic<State>&>(*self);