mkdir.2: Glibc falls back to mkdir() on kernels that don't support mkdirat()

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

man2/mkdir.2

@@ -220,6 +220,19 @@ See also
 There are many infelicities in the protocol underlying NFS.
 Some of these affect
 .BR mkdir ().
+.SS Glibc notes
+On older kernels where
+.BR mkdirat ()
+is unavailable, the glibc wrapper function falls back to the use of
+.BR mkdir ().
+When
+.I pathname
+is a relative pathname,
+glibc constructs a pathname based on the symbolic link in
+.IR /proc/self/fd
+that corresponds to the
+.IR dirfd
+argument.
 .SH SEE ALSO
 .BR mkdir (1),
 .BR chmod (2),