Add permission note to runners.md

doc/api/runners.md

@@ -156,6 +156,10 @@ Example response:
 
 Get details of a runner.
 
+[Maintainer access or higher](../user/permissions.md) is required to get runner details at the project and group level. 
+
+Instance-level runner details via this endpoint are available to all signed in users.
+
 ```plaintext
 GET /runners/:id
 ```