Re-add "Update configuration files" section in the 8.4 to 8.5 upgrade guide

Needed because of !2782.

[ci skip]

doc/update/8.4-to-8.5.md

@@ -72,12 +72,22 @@ sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS
 
 ```
 
-### 7. Start application
+### 7. Update configuration files
+
+#### New configuration options for `gitlab.yml`
+
+There are new configuration options available for [`gitlab.yml`](config/gitlab.yml.example). View them with the command below and apply them manually to your current `gitlab.yml`:
+
+```sh
+git diff origin/8-4-stable:config/gitlab.yml.example origin/8-5-stable:config/gitlab.yml.example
+```
+
+### 8. Start application
 
     sudo service gitlab start
     sudo service nginx restart
 
-### 8. Check application status
+### 9. Check application status
 
 Check if GitLab and its environment are configured correctly: