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>

src/core/api/qwebenginequotapermissionrequest.cpp

@@ -50,8 +50,11 @@ QT_BEGIN_NAMESPACE
     in File System API.
 
     \since 5.11
-
     \inmodule QtWebEngineCore
+
+    This class is used by the QWebEnginePage::quotaPermissionRequested() signal
+    to \l accept() or \l reject() a request for an increase in the persistent
+    storage allocated to the application. The default quota is 0 bytes.
 */
 
 /*! \fn QWebEngineQuotaPermissionRequest::QWebEngineQuotaPermissionRequest()