Add troubleshooting for GitLab Pages listen proxy

doc/administration/pages/index.md

@@ -720,6 +720,23 @@ sudo cp /opt/gitlab/embedded/ssl/certs/cacert.pem /var/opt/gitlab/gitlab-rails/s
 sudo cp /opt/gitlab/embedded/ssl/certs/cacert.pem /var/opt/gitlab/gitlab-rails/shared/pages/etc/ssl/ca-bundle.pem
 ```
 
+### 502 error when connecting to GitLab Pages proxy when server does not listen over IPv6
+
+In some cases, NGINX might default to using IPv6 to connect to the GitLab Pages
+proxy even when the server does not listen over IPv6. You can identify when
+this is happening if you see something similar to the log entry below in the
+`gitlab_pages_error.log`:
+
+```plaintext
+2020/02/24 16:32:05 [error] 112654#0: *4982804 connect() failed (111: Connection refused) while connecting to upstream, client: 123.123.123.123, server: ~^(?<group>.*)\.pages\.example\.com$, request: "GET /-/group/project/-/jobs/1234/artifacts/artifact.txt HTTP/1.1", upstream: "http://[::1]:8090//-/group/project/-/jobs/1234/artifacts/artifact.txt", host: "group.example.com"
+```
+
+To resolve this, set an explicit IP and port for the GitLab Pages listen proxy:
+
+```ruby
+gitlab_pages['listen_proxy'] = '127.0.0.1:8090'
+```
+
 ### 404 error after transferring project to a different group or user
 
 If you encounter a `404 Not Found` error a Pages site after transferring a project to