Document the fact that QWebEngineView uses QQuickWidget internally

QWebEngineView's internal QQuickWidget is here to stay (see the outcome of QTBUG-85236). So, it would be helpful for our documentation to reveal its existence.

This would help users understand why things break if, say, they put a QOpenGLWidget and a QWebEngineView in the same window on Windows/macOS (since QQuickWidget would try to use Direct3D11 on Windows which is incompatible with OpenGL – we need to call QQuickWindow::setGraphicsApi(QSGRendererInterface::OpenGL) to let the pair coexist happily)