nginx: add listen note

src/nginxconfig.ejs

@@ -12,6 +12,7 @@ map $upstream_http_referrer_policy $hrp {
 
 # http server
 server {
+    # note listen [::]:80 only listens on ipv6 since ipv6only=on since nginx 1.3.4. listen 80 listens on ipv4 only
 <% if (endpoint === 'ip' || endpoint === 'setup') { -%>
     listen 80 default_server;
     server_name _;