Restore the docs for QFlags debug streaming operator

The unconstrained implementation was removed, so qdoc was not able to
find a proper function. Provide a fake definition that matches what
qdoc expects. In this case the users shouldn't really care about the
constraints that the actual implementation has.

Amends 0c707c6ebece18677deaccccdfec5d48c587106f.

Found in Qt 6.9 API review.

Pick-to: 6.9
Change-Id: I6207b491fa94cc62d217af7dc7687923dc6971a4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qdebug.h

@@ -655,6 +655,11 @@ inline QDebug operator<<(QDebug debug, Flags flags)
     }
 }
 
+#ifdef Q_QDOC
+template <typename T>
+QDebug operator<<(QDebug debug, const QFlags<T> &flags);
+#endif // Q_QDOC
+
 #if !defined(QT_NO_QOBJECT) && !defined(Q_QDOC)
 // Debugging of plain enums. There are three cases:
 //  1) the enum is part of a Q_DECLARE_FLAGS and there's a Q_FLAG for that