MenuItemDescription

Hi,
i have a SplitButton in my ApplicationMenu like so:

<ribbon:SplitButton x:Name="xbsPrintButton"
Label="Dokument erstellen"
KeyTipAccessText="D"
MenuItemDescription="Erstellt das Standarddokument"
Command="ApplicationCommands.Print"
VariantSize="Large"
>

How can i enable the display of the MenuItemDescription (with the bold Label above, which is working fine for SplitButtons that are part of a "regular" menu)?

Regards

Michael