Updating sql style guide to add additional comment for using datediff
As related to !37006 (04979c85)
Using the correct interval will help ensure we are capturing what is needed
SELECT DATEDIFF('days', '2001-12-01 23:59:59.999', '2001-12-02 00:00:00.000'),
DATEDIFF('ms', '2001-12-01 23:59:59.998', '2001-12-02 00:00:00.000')