Added a basic example

[yashints]

Summary

Added a basic example of the usage

Fixes dotnet/docs#6272

[dnfclas]

All CLA requirements met.

[yashints]

@BillWagner could you review the changes now? included a wrong use case and a unit test as you mentioned

[BillWagner]

I've been remiss in commenting on this PR; I apologize for that.

The delay is that I'm struggling with the right recommendation to update it. In general, we try to write positive guidance rather than negative guidance. There's two reasons:

1. A reader seeing "don't do this" asks the obvious question "what should I do instead?"
2. APIs are created for a reason. Our role with docs is to identify those scenarios where an API should be used (and direct readers to alternatives when they land at the wrong solution).

I'm checking with the team to get recommendations.

[BillWagner]

After a lot of debate, I'm closing this.

The API reference is the wrong location for conceptual guidance, which is what this PR does. That means developers find this only when they've already reach the point where they are searching for a specific method.

A better way to provide this guidance is to create an article in our conceptual section that provides the correct way to ensure that a GUI remains responsive during long-running tasks.

Consider this type of guidance as part fo dotnet/docs#6373 or dotnet/docs#951