Document clear behaviour of list options

More general for all list options.

Seems to currently affect:
lxc.network (clear all NICs)
lxc.network.* (clear current NIC)
lxc.cap.drop
lxc.cap.keep
lxc.cgroup
lxc.mount.entry
lxc.mount.auto
lxc.hook
lxc.id_map
lxc.group
lxc.environment

Signed-off-by: Marko Hauptvogel <marko.hauptvogel@googlemail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>

doc/lxc.container.conf.sgml.in

@@ -72,7 +72,9 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
     <para>
       Each option in the configuration file has the form <command>key
       = value</command> fitting in one line. The '#' character means
-      the line is a comment.
+      the line is a comment. List options, like capabilities and cgroups
+      options, can be used with no value to clear any previously
+      defined values of that option.
     </para>
 
     <refsect2>