Merge branch 'docs/ssot_external_auth' into 'master'

Edit external auth for SSOT

See merge request gitlab-org/gitlab-ce!28630

doc/user/admin_area/settings/external_authorization.md

+---
+type: reference
+---
+
 # External authorization control **[CORE ONLY]**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/4216) in
@@ -108,5 +112,17 @@ The label will be shown on all project pages in the upper right corner.
 
 ![classification label on project page](img/classification_label_on_project_page.png)
 
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->
+
 [omnibus-ssl-docs]: https://docs.gitlab.com/omnibus/settings/ssl.html
 [omnibus-log-docs]: https://docs.gitlab.com/omnibus/settings/logs.html