Correctly handle one-line classes in doc check

Fixes #71

Also fixes this case that was not mentioned:

```ruby
class RequiresDoc
  class AlsoRequiresDoc; def foo; end; end
  class DoesNotRequireDoc; end

  def bar; end
end
```

We want both `RequiresDoc` and `AlsoRequiresDoc` to have docs.

The code gets a bit more complicated with these special cases, but is hopefully still easy to follow. I'm trying my best to avoid building some giant AST :grimacing: