Update docs for QButtonGroup

In the Detailed Description section, it's says

The id of the currently checked button is available with checkedId(), and there is an overloaded signal buttonClicked() which emits the id of the button.

I'm not convinced this is true. That is, I can't figure out how to access this signal. What definitely is available is idClicked. So I propose that documentation be changed to read:
noformat}
The id of the currently checked button is available with checkedId(), and there is a signal idClicked() which emits the id of the button.