[PATCH] manual: Add Descriptor-Relative Access section

[Florian Weimer]

* Florian Weimer:

> 8<------------------------------------------------------------------8<
> 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.)

Could I please get a formal review for this patch?  Thanks.

  <https://sourceware.org/pipermail/libc-alpha/2020-May/113554.html>