fix(docs): added api_url to configuration docs, fixes #6351

docs/sources/installation/configuration.md

@@ -392,9 +392,12 @@ browser to access Grafana, but with the prefix path of `/login/generic_oauth`.
     scopes =
     auth_url =
     token_url =
+    api_url =
     allowed_domains = mycompany.com mycompany.org
     allow_sign_up = false
 
+Set api_url to the resource that returns basic user info.
+
 <hr>
 
 ## [auth.basic]