doc: Update for glibc 2.30.
* doc/glibc-functions/gettid.texi: New file.
* doc/glibc-functions/pthread_cond_clockwait.texi: New file.
* doc/glibc-functions/pthread_mutex_clocklock.texi: New file.
* doc/glibc-functions/pthread_rwlock_clockrdlock.texi: New file.
* doc/glibc-functions/pthread_rwlock_clockwrlock.texi: New file.
* doc/glibc-functions/sem_clockwait.texi: New file.
* doc/glibc-functions/tgkill.texi: New file.
* doc/glibc-functions/twalk_r.texi: New file.
* doc/gnulib.texi: Include them.
(Glibc semaphore.h): New section.
* doc/pastposix-functions/h_errno.texi: Update.
* doc/posix-functions/*.texi: Likewise.