Clarify ownership around QQmlEngine::setNAMF

We were leaking an instance of MyNetworkAccessManagerFactory in the
"NetworkAccessManagerFactory"-example. To add to this the documentation
around QQmlEngine::setNetworkAccessManagerFactory did not specify
whether or not it took ownership, causing confusion.

Change-Id: Ic9eee2c45682c752bcb4aa98943fc0af2b630795
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

examples/qml/networkaccessmanagerfactory/main.cpp

@@ -133,7 +133,8 @@ int main(int argc, char ** argv)
         source = QUrl::fromLocalFile(parser.positionalArguments().first());
 
     QQuickView view;
-    view.engine()->setNetworkAccessManagerFactory(new MyNetworkAccessManagerFactory);
+    MyNetworkAccessManagerFactory networkManagerFactory;
+    view.engine()->setNetworkAccessManagerFactory(&networkManagerFactory);
 
     view.setSource(source);
     view.show();

src/qml/qml/qqmlengine.cpp

@@ -1196,6 +1196,8 @@ void QQmlEnginePrivate::registerFinalizeCallback(QObject *obj, int index)
   support.
 
   The factory must be set before executing the engine.
+
+  \note QQmlEngine does not take ownership of the factory.
 */
 void QQmlEngine::setNetworkAccessManagerFactory(QQmlNetworkAccessManagerFactory *factory)
 {