doc: Mention the ISO C11 multithreading header and functions.
* doc/posix-headers/threads.texi: New file.
* doc/posix-functions/call_once.texi: New file.
* doc/posix-functions/cnd_broadcast.texi: New file.
* doc/posix-functions/cnd_destroy.texi: New file.
* doc/posix-functions/cnd_init.texi: New file.
* doc/posix-functions/cnd_signal.texi: New file.
* doc/posix-functions/cnd_timedwait.texi: New file.
* doc/posix-functions/cnd_wait.texi: New file.
* doc/posix-functions/mtx_destroy.texi: New file.
* doc/posix-functions/mtx_init.texi: New file.
* doc/posix-functions/mtx_lock.texi: New file.
* doc/posix-functions/mtx_timedlock.texi: New file.
* doc/posix-functions/mtx_trylock.texi: New file.
* doc/posix-functions/mtx_unlock.texi: New file.
* doc/posix-functions/thrd_create.texi: New file.
* doc/posix-functions/thrd_current.texi: New file.
* doc/posix-functions/thrd_detach.texi: New file.
* doc/posix-functions/thrd_equal.texi: New file.
* doc/posix-functions/thrd_exit.texi: New file.
* doc/posix-functions/thrd_join.texi: New file.
* doc/posix-functions/thrd_sleep.texi: New file.
* doc/posix-functions/thrd_yield.texi: New file.
* doc/posix-functions/tss_create.texi: New file.
* doc/posix-functions/tss_delete.texi: New file.
* doc/posix-functions/tss_get.texi: New file.
* doc/posix-functions/tss_set.texi: New file.
* doc/gnulib.texi (Header File Substitutes): Include posix-headers/threads.texi.
(Function Substitutes): Include the new posix-functions/*.texi files.