Add note about confidential trusted OAuth apps

app/views/admin/applications/_form.html.haml

@@ -23,7 +23,7 @@
     .col-sm-10
       = f.check_box :trusted
       %span.form-text.text-muted
-        Trusted applications are automatically authorized on GitLab OAuth flow.
+        Trusted applications are automatically authorized on GitLab OAuth flow. It's highly recommended for the security of users that trusted applications have the confidential setting set to true.
 
   = content_tag :div, class: 'form-group row' do
     .col-sm-2.col-form-label.pt-0