cgroups: clarify comment about when dumping criu cgset

Empty cgroups will not be preserved in the criu cgset, since we don't ever
dump the criu cgset. This is perhaps a corner case that we may want to
address, but is probably (?) really only relevant for test cases.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>