Documentation for QToolButton styling

Feedback from Osama K (via feedback@qt.io):

Currently, the documentation does not include anything for styling the menu of a QToolButton (with popup mode MenuButtonPopup) through the style sheets. It’s not clear if the menu has to be coded in a .cpp file or through some other method. I’d generally prefer not to code to minimize chances of errors occurring through code whenever possible, so it would be great if you could include documentation for a QMenu under a QToolButton.