Update conf/scripts/route-up.d/40-vpnclient-set-ipv6

Co-authored-by: Alexandre Aubin <4533074+alexAubin@users.noreply.github.com>

conf/scripts/route-up.d/40-vpnclient-set-ipv6

@@ -6,6 +6,7 @@ is_ip6addr_set() {
 }
 
 ip6_addr=$(yunohost app setting "vpnclient" "ip6_addr")
+# cf https://openvpn.net/community-resources/reference-manual-for-openvpn-2-4/#environmental-variables for where 'dev' comes from
 gateway_interface=${dev}
 
 if [[ -n "${ip6_addr}" ]] && [[ "${ip6_addr}" != none ]]; then