documentation: Add missing documentation for pthread functions and structures

As wished in #980.

WIP:
- ~~pthread.h~~
- ~~pthread_barrier.h~~
- ~~pthread_cancellation.h~~
- ~~pthread_cleanup.h~~
- pthread_cond.h ← blocked by #771 
- ~~pthread_mutex.h~~
- ~~pthread_mutex_attr.h~~
- ~~pthread_once.h~~
- pthread_rwlock.h ← blocked by #1018 
- ~~pthread_spin.h~~
- ~~pthread_scheduling.h~~
- ~~pthread_threading.h~~
- ~~pthread_threading_attr.h~~