Update New-Label.md with Date-Time variable for DW

Author: cabirch

exchange/exchange-ps/exchange/New-Label.md

@@ -722,6 +722,7 @@ Accept wildcard characters: False
 The DynamicWatermarkDisplay parameter specifies the watermark text to display for a given label. This parameter supports text and the following special tokens:
 
 - `${Consumer.PrincipalName}`: Required. The value is the user principal name (UPN) of the user.
+- `${Device.DateTime}`: Optional. The value is current date/time of the device used to view the document.
 
 This parameter is meaningful only when the ApplyDynamicWatermarkingEnabled parameter value is $true.