I was talking with @tvolkert about the complex behavior of Shortcuts when a text field is focused.  I created [this dartpad](https://dartpad.dev/?id=0b5c08fa85637422baa84927b7f1ee5f) to illustrate the problem, which shows a key being stolen from a text field by Shortcuts, and how to prevent that using DoNothingAndStopPropagationIntent.

This PR adds a section in the docs explaining how all of this works and how to override this "stealing" problem.