Mark QPropertyAlias as internal

That class was only really meant for the QML compiler, and it is dubious
if event that one needs it. The current implementation is also broken.

Pick-to: 6.1 6.0
Change-Id: Ie40d282707f3fabc8079bee9e98f082aeb9d30b3
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -1338,6 +1338,8 @@ QString QPropertyBindingError::description() const
 /*!
   \class QPropertyAlias
   \inmodule QtCore
+  \internal
+
   \brief The QPropertyAlias class is a safe alias for a QProperty with same template parameter.
 
   \ingroup tools