specify milliseconds when subtracting dates

Author: MongoCaleb

source/reference/operator/aggregation/subtract.txt

@@ -65,7 +65,8 @@ Subtract Two Dates
 ~~~~~~~~~~~~~~~~~~
 
 The following aggregation uses the :expression:`$subtract` expression
-to subtract ``$date`` from the current date:
+to subtract ``$date`` from the current date and returns the difference 
+in milliseconds:
 
 .. code-block:: javascript