Visual Studio automatically enables IntelliSense - it detects all classes, properties and methods in the DLL file.
When declaring a namespace you can select from the IntelliSense in XAML any namespace which is available:
For more information about declaring namespaces read here.
When you add a new control (or configure an existing one) in the XAML, Visual Studio will automatically show a dropdown with suggestions for autocomplete. For example, see the next two figures.
In the code-behind, each time a particular class/property/method is being selected from the dropdown of the autocomplete functionality, a tooltip with the comment for this class/property/method is displayed.