Add a comment to highlight QSpontaneKeyEvent usage in QtWebEngine

- QtWebEngine event handling requires forwarding events as spontaneous.
- Impersonated QSpontaneKeyEvent in QtWebEngine to handle such cases.
- Added comment to explain QSpontaneKeyEvent usage in QtWebEngine.

Tasks: QTBUG-118398

Change-Id: I0ef951802727065bed660210e407a5a830956adf
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qcoreevent.h

@@ -354,6 +354,8 @@ private:
     friend class QApplication;
     friend class QGraphicsScenePrivate;
     // from QtTest:
+    // QtWebEngine event handling requires forwarding events as spontaneous.
+    // Impersonated QSpontaneKeyEvent in QtWebEngine to handle such cases.
     friend class QSpontaneKeyEvent;
     // needs this:
     Q_ALWAYS_INLINE