manual: Add Descriptor-Relative Access section

manual: Add Descriptor-Relative Access section

Reference this new section from the O_PATH documentation.

And document the functions openat, openat64, fstatat, fstatat64.
(The safety assessment for fstatat was already obsolete because
current glibc assumes kernel support for the underlying system
call.)

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>