Add a note about the bug the previous workaround was for

src/calibre/gui2/main_window.py

@@ -164,6 +164,9 @@ class MainWindow(QMainWindow):
         return QMainWindow.event(self, ev)
 
 def clone_menu(menu):
+    # This is needed to workaround a bug in Qt 5.5+ and Unity. When the same
+    # QAction object is used in both a QMenuBar and a QMenu, sub-menus of the
+    # QMenu flicker when rendered under Unity.
     def clone_action(ac, parent):
         if ac.isSeparator():
             ans = QAction(parent)