Add more details to Redirect URI troubleshooting

doc/administration/pages/index.md

@@ -1332,8 +1332,11 @@ Once added, reconfigure with `sudo gitlab-ctl reconfigure` and restart GitLab wi
 
 ### `The redirect URI included is not valid.` when using Pages Access Control
 
-Verify that the **Callback URL**/Redirect URI in the GitLab Pages [System OAuth application](../../integration/oauth_provider.md#instance-wide-applications)
+You may see this error if `pages_external_url` was updated at some point of time. Verify the following:
+
+1. The **Callback URL**/Redirect URI in the GitLab Pages [System OAuth application](../../integration/oauth_provider.md#instance-wide-applications)
 is using the protocol (HTTP or HTTPS) that `pages_external_url` is configured to use.
+1. The domain and path components of `Redirect URI` are valid: they should look like `projects.<pages_external_url>/auth`.
 
 ### 500 error `cannot serve from disk`