Add note about letters

When trying to send letters using the API, the ‘team and whitelist’ key
is confusing. We don’t have addresses for your team members, nor is
there a whitelist for letter addresses. The actual behaviour is that
you’ll get an error if you try to use this key to send letters.

So, for services who have letters available, we should add a hint
telling users that team and whitelist is probably not the key they’re
looking for.

tests/conftest.py

@@ -268,6 +268,19 @@ def mock_get_live_service(mocker, api_user_active):
     return mocker.patch('app.service_api_client.get_service', side_effect=_get)
 
 
+@pytest.fixture(scope='function')
+def mock_get_service_with_letters(mocker, api_user_active):
+    def _get(service_id):
+        return {'data': service_json(
+            service_id,
+            users=[api_user_active.id],
+            restricted=False,
+            permissions=['email', 'sms', 'letter']
+        )}
+
+    return mocker.patch('app.service_api_client.get_service', side_effect=_get)
+
+
 @pytest.fixture(scope='function')
 def mock_create_service(mocker):
     def _create(service_name, message_limit, restricted, user_id, email_from):