Errors in AppMan NotificationManager docs

This page needs updating: https://doc.qt.io/QtApplicationManager/qml-qtapplicationmanager-systemui-notificationmanager.html

• The documentation refers to an "isClickable" role, which doesn't exist
• The "acknowledgeable" role is missing (see https://doc.qt.io/QtApplicationManager/qml-qtapplicationmanager-notification.html#acknowledgeable-prop )
• "freedesktop.org specification" (external link) lead to Error 404