Document when milestones and labels links are missing

Document that milestones and labels are still accesible
through direct links when issues are disabled even if
links are missing in sidebar UI.

changelogs/unreleased/58984-doc-missing-milestones-and-labels-links.yml

+---
+title: Document when milestones and labels links are missing
+merge_request: 29355
+author:
+type: other

doc/user/project/settings/index.md

@@ -26,6 +26,12 @@ Set up your project's access, [visibility](../../../public_access/public_access.
 
 ![projects sharing permissions](img/sharing_and_permissions_settings.png)
 
+If Issues are disabled, or you can't access Issues because you're not a project member, then Lables and Milestones 
+links will be missing from the sidebar UI.
+
+You can still access them with direct links if you can access Merge Requests. This is deliberate, if you can see 
+Issues or Merge Requests, both of which use Labels and Milestones, then you shouldn't be denied access to Labels and Milestones pages.
+
 ### Issue settings
 
 Add an [issue description template](../description_templates.md#description-templates) to your project, so that every new issue will start with a custom template.