These warning messages aren't very helpful. 

```
(eliot)(better-deferred-api-13 =)[~/.../HybridLogic/eliot]$ PYTHONWARNINGS=all trial eliot
eliot.tests.test_action
  ActionTests
...
    test_runCallback ... /home/richard/projects/HybridLogic/eliot/eliot/tests/test_action.py:286: DeprecationWarning: Use eliot.twisted instead.
```
Can you at least add a reference to the deprecated API and maybe a specific API within eliot.twisted

Also, if this were Twisted, we'd require tests for these deprecation messages.