Doc: Describe use of QWebEngineQuotaPermissionRequest class

It is only used by the QWebEnginePage::quotaPermissionRequested
signal.

Task-number: QTBUG-66779
Change-Id: I265cc1787a5470080706fee548c41624c741e540
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>