Add comment explaining mocking

> I'd find it useful to know mock_get_jobs mocks
> app.job_api_client.get_jobs here, perhaps through a comment. Reading
> it, I associated it with contact_list.get_jobs above.

tests/app/models/test_contact_list.py

@@ -13,6 +13,8 @@ def test_created_at():
 def test_get_jobs(mock_get_jobs):
     contact_list = ContactList({'id': 'a', 'service_id': 'b'})
     assert isinstance(contact_list.get_jobs(page=123), PaginatedJobs)
+    # mock_get_jobs mocks the underlying API client method, not
+    # contact_list.get_jobs
     mock_get_jobs.assert_called_once_with(
         'b',
         contact_list_id='a',