Coding style: update advice

to do not use unique_ptr instead of auto_ptr which was deprecated

Change-Id: If6e5bd25c62184a5d34c0cdedf0206138e32bf28
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>

doc/api/coding-style.qdoc

@@ -1239,7 +1239,7 @@
                 int value() const;
 
                 QScopedPointer<BarPrivate> d;
-                //std::auto_ptr<BarPrivate> d;
+                //std::unique_ptr<BarPrivate> d;
             };
 
             ############### bar.cpp