LXC: add securetty related note in Device nodes

Tell user how to resolve the problem that fail to log in
the container.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>

docs/drvlxc.html.in

@@ -163,6 +163,14 @@ first console will be <code>/dev/tty1</code>, with further consoles
 numbered incrementally from there.
 </p>
 
+<p>
+Since /dev/ttyN and /dev/console are linked to the pts devices. The
+tty device of login program is pts device. the pam module securetty
+may prevent root user from logging in container. If you want root
+user to log in container successfully, add the pts device to the file
+/etc/securetty of container.
+</p>
+
 <p>
 Further block or character devices will be made available to containers
 depending on their configuration.