update overlayfs and aufs in lxc.container.conf

doc/lxc.container.conf.sgml.in

@@ -1048,7 +1048,9 @@ proc proc proc nodev,noexec,nosuid 0 0
           specifies that the rootfs should be an overlay with <filename>/upper</filename>
           being mounted read-write over a read-only mount of <filename>/lower</filename>.
           <filename>aufs:/lower:/upper</filename> does the same using aufs in place
-          of overlayfs. <filename>loop:/file</filename> tells lxc to attach
+          of overlayfs. For both <filename>overlayfs</filename> and
+          <filename>aufs</filename> multiple <filename>/lower</filename>
+          directories can be specified. <filename>loop:/file</filename> tells lxc to attach
           <filename>/file</filename> to a loop device and mount the loop device.
             </para>
           </listitem>