This is an example on how to use Roslyn to support other parts of your job. Specifically, it can be used to create a text representation of a class diagram from your source code. This representation can then be used by PlantUML to create an image. The advantage of this approach is that the class diagrams can be generated programmatically and can be integrated in the systems of continuous integration that you already use (or your git/mercurial repository).
You can read an article on the example on Generate diagrams from C# source code