Type docs applied to all partial classes

https://github.com/dotnet/winforms has multiple types that span across multiple files. The tool is currently applying type xmldocs to all partial definitions.

Instead we should track whether a type has been annotated, and not annotate other partials. When deduping we should perhaps look for the file with the shortest name, e.g. from the following list pick `Application.cs`:
```
src/System.Windows.Forms/src/System/Windows/Forms/Application.ComponentManager.cs
src/System.Windows.Forms/src/System/Windows/Forms/Application.cs
src/System.Windows.Forms/src/System/Windows/Forms/Application.MarshallingControl.cs
src/System.Windows.Forms/src/System/Windows/Forms/Application.ModalApplicationContext.cs
src/System.Windows.Forms/src/System/Windows/Forms/Application.ParkingWindow.cs
src/System.Windows.Forms/src/System/Windows/Forms/Application.ThreadContext.cs
src/System.Windows.Forms/src/System/Windows/Forms/Application.ThreadWindows.cs
```