Include instructions to update /etc/default/gitlab

We were missing some info on updating /etc/default/gitlab

In particular, changes needed to be made in order for Pages to work,
see https://gitlab.com/gitlab-org/gitlab-ce/issues/29372

doc/update/8.16-to-8.17.md

@@ -139,7 +139,7 @@ sudo -u git -H git checkout v4.1.1
 
 #### 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`:
+There might be new configuration options available for [`gitlab.yml`][yaml]. View them with the command below and apply them manually to your current `gitlab.yml`:
 
 ```sh
 cd /home/git/gitlab
@@ -195,6 +195,16 @@ See [smtp_settings.rb.sample] as an example.
 
 #### Init script
 
+There might be new configuration options available for [`gitlab.default.example`][gl-example].
+You need to update this file if you want to [enable GitLab Pages][pages-admin].
+View them with the command below and apply them manually to your current `/etc/default/gitlab`:
+
+```sh
+cd /home/git/gitlab
+
+git diff origin/8-16-stable:lib/support/init.d/gitlab.default.example origin/8-17-stable:lib/support/init.d/gitlab.default.example
+```
+
 Ensure you're still up-to-date with the latest init script changes:
 
 ```bash
@@ -252,3 +262,7 @@ sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
 ```
 
 If you have more than one backup `*.tar` file(s) please add `BACKUP=timestamp_of_backup` to the command above.
+
+[yaml]: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-17-stable/config/gitlab.yml.example
+[gl-example]: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-17-stable/lib/support/init.d/gitlab.default.example
+[pages-admin]: ../administration/pages/source.md