linkat.2: Add ENOENT for O_TMPFILE created with O_EXCL

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/linkat.2

@@ -172,6 +172,16 @@ but the caller did not have the
 .B CAP_DAC_READ_SEARCH
 capability.
 .TP
+.B ENOENT
+An attempt was made to link to the
+.I /proc/self/fd/NN
+file corresponding to a file descriptor created with
+
+    open(path, O_TMPFILE | O_EXCL, mode);
+
+See
+.BR open (2).
+.TP
 .B ENOTDIR
 .I oldpath
 is relative and