lxc-commands: add a comment explaining CMD_* rules

We wish to ensure that, henceforth, newer lxc tools are always compatible
with older lxc monitors.  Add a comment to commands.c to explain the
rule we wish to enforce to this end.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>