criu(8): describe --external option
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 <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu