doc: update timer_* platform portability notes * doc/posix-functions/timer_create.texi: Add platforms that always return ENOSYS (OpenBSD 4.9). * doc/posix-functions/timer_delete.texi: Likewise. * doc/posix-functions/timer_gettime.texi: Likewise. * doc/posix-functions/timer_settime.texi: Likewise.