special pages config only needed for custom domain support

doc/administration/high_availability/load_balancer.md

@@ -18,7 +18,8 @@ you need to use with GitLab.
 ## GitLab Pages Ports
-If you're using GitLab Pages you will need some additional port configurations.
+If you're using GitLab Pages with custom domain support you will need some 
+additional port configurations.
 GitLab Pages requires a separate virtual IP address. Configure DNS to point the
 `pages_external_url` from `/etc/gitlab/gitlab.rb` at the new virtual IP address. See the
 [GitLab Pages documentation][gitlab-pages] for more information.