ExtendedTask: add example to docs

[daattali]

The docs for this function don't have an example or even a link to the article on it. I copied the example from bind_task_button()

[daattali]

I only added the roxygen comment, I did not compile into Rd

[daattali]

This will fix https://github.com/rstudio/shiny/issues/4024

[gadenbuie]

Thanks @daattali!