doc: tweak *at function documentation

doc: tweak *at function documentation

* doc/posix-functions/faccessat.texi (faccessat): Mention
known issue with replacement.
* doc/posix-functions/fchdir.texi (fchdir): Likewise.
* doc/posix-functions/linkat.texi (linkat): Likewise.
* doc/posix-functions/mkfifoat.texi (mkfifoat): Likewise.
* doc/posix-functions/mknodat.texi (mknodat): Likewise.
* doc/posix-functions/readlinkat.texi (readlinkat): Likewise.
* doc/posix-functions/renameat.texi (renameat): Likewise.
* doc/posix-functions/symlinkat.texi (symlinkat): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>