QPrintDialog: document the modality on OS X and Windows

You can't programmatically close the dialog because it's modal.

Task-number: QTBUG-32464
Change-Id: I2f24581dc660a088b4e741fa4ee1518e27adea4a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/printsupport/dialogs/qabstractprintdialog.cpp

@@ -460,6 +460,10 @@ void QAbstractPrintDialog::setOptionTabs(const QList<QWidget*> &tabs)
   is shown with exec(), done() causes the local event loop to finish,
   and exec() to return \a result.
 
+  \note This function does not apply to the Native Print Dialog on the Mac
+  OS X and Windows platforms, because the dialog is required to be modal
+  and only the user can close it.
+
   \sa QDialog::done()
 */
 void QPrintDialog::done(int result)