Customizing Ctrl-Hover popup for references

Hi,

For a custom language, I'd like to customize the Ctrl-hover popup.
What's the extension point to hook into?
And what's the implementation class (in intellij-community) that's doing this for the Java language?
(to clarify, I'm referring to the component that's rendering the base class, module and other information in the small yellow tooltip when doing a Ctrl-hover over a Java reference).

Thanks,
Taras