Remove async void usage from "How to Access the UI Thread"

[stevemonaco]

How to Access the UI Thread

private async void OnTextFromAnotherThread(string text) is not an event handler.

The fire-and-forget in the ctor should probably be moved into Loaded so it can be awaited:
_ = Task.Run(() => OnTextFromAnotherThread("test"));

Docs should encourage safe patterns where possible.