Declare the QMetaType::WasDeclaredAsMetaType enum as private

Omit its value in the documentation.

Change-Id: I474faefde23b0e2e0a77a9e6391fd556a6523128
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/corelib/kernel/qmetatype.cpp

@@ -240,6 +240,7 @@ struct DefinedTypesFilter {
     \omitvalue PointerToQObject
     \omitvalue WeakPointerToQObject
     \omitvalue TrackingPointerToQObject
+    \omitvalue WasDeclaredAsMetaType
 */
 
 /*!