Edited signup restrictions for SSOT guidelines

doc/user/admin_area/settings/sign_up_restrictions.md

+---
+type: reference
+---
+
 # Sign-up restrictions
 
 You can block email addresses of specific domains, or whitelist only some
@@ -37,5 +41,17 @@ semicolon, comma, or a new line.
 
 ![Domain Blacklist](img/domain_blacklist.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. -->
+
 [ce-5259]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5259
 [ce-598]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/598