docs: signout_redirect_url description in auth overview

docs/sources/auth/overview.md

@@ -95,3 +95,12 @@ Set to the option detailed below to true to hide sign-out menu link. Useful if y
 [auth]
 disable_signout_menu = true
 ```
+
+### URL redirect after signing out
+
+URL to redirect the user to after signing out from Grafana. This can for example be used to enable signout from oauth provider.
+
+```bash
+[auth]
+signout_redirect_url =
+```