help: update help output for --cgroup-root

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/crtools.c

@@ -859,8 +859,11 @@ usage:
 "  --manage-cgroups [m]  dump/restore process' cgroups; argument can be one of\n"
 "                        'none', 'props', 'soft' (default), 'full' or 'strict'\n"
 "  --cgroup-root [controller:]/newroot\n"
-"                        change the root cgroup the controller will be\n"
-"                        installed into. No controller means that root is the\n"
+"                        on dump: change the root for the controller that will\n"
+"                        be dumped. By default, only the paths with tasks in\n"
+"                        them and below will be dumped.\n"
+"                        on restore: change the root cgroup the controller will\n"
+"                        be installed into. No controller means that root is the\n"
 "                        default for all controllers not specified\n"
 "  --cgroup-props STRING\n"
 "                        define cgroup controllers and properties\n"