Document changes in touch events and points

Since Qt6 we are getting rid of QTouchEvent::QTouchPoint and adapting QEventPoint. But it is not an equivalent. Back in Qt5, people can just make up touch points and then simulate touch events. But in Qt6, it is not possible. QEventPoint is pretty much read-only and there are just no setters for users to create valid touch points and simulate touch events (Constructor accepts 2 positions, yes. But that is pretty much it. No setters for all the other properties).

The main concern here is that we should document that in:
https://doc.qt.io/qt-6/gui-changes-qt6.html
And optionally, also put some reasonings there about why we are limiting the use cases and additional suggestions on porting Qt5 touch-related codes to Qt6.