criu(8): describe --external option

This adds the description of --external option for all the supported
cases, both for dump and restore.

References: https://criu.org/CLI/opt/--external



travis-ci: success for Add/fix description of --external and --inherit-fd
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>