I was thinking it might be good to add a paragraph here such as:

```
Note: `class_double` can be used for modules as well. We chose to stick to the
`class_double` terminology because the methods a `class_double` verifies against
are commonly called "class methods", not "module methods", even when working
with a module.
```

Thoughts?  Does that express the reasons we don't have a `module_double` method well?