Doc: QPicture uses QDataStream, forward warning against untrusted data

Make this explicit, with a link.

Pick-to: 6.9 6.8
Change-Id: I8b8b64c88daacba43df1eecf5a347c0a89d837e1
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/image/qpicture.cpp

@@ -69,6 +69,10 @@ void qt_format_text(const QFont &fnt, const QRectF &_r,
     picture is available, for example, size(), isNull() and
     boundingRect().
 
+    \note QPicture uses QDataStream for serialization. The
+    \l {QDataStream#Corruption and Security}{same reservations} against
+    reading untrusted data apply.
+
     \sa QMovie
 */