The QuickDoc feature displays a popup window with details of the code element at the current text caret position. What gets displayed depends on what that code element is.
For example, a C# QuickDoc window will show the element name and type, with hyperlinks to navigate to any type definitions and description from XML documentation.
A CSS QuickDoc window will show the expected values for the property, with hyperlinks to the definitions of the value units, and a "read more" link to see the W3C website's documentation.
Implement your own support, by implementing IQuickDocProvider and IQuickDocPresenter, using the helper classes