docs: add explanation for argument in datediff macro

README.md

@@ -66,7 +66,7 @@ Usage:
 `{{ pm_utils.date_from_timestamp('[expression]') }}`
 
 #### datediff ([source](macros/multiple_databases/datediff.sql))
-This macro computes the difference between two date, time, or datetime expressions based on the specified datepart. It returns an integer indicating the difference in, for example, milliseconds, days, or months.
+This macro computes the difference between two date, time, or datetime expressions based on the specified `datepart` and returns an integer value. The datepart can be any of the following values: year, quarter, month, week, day, hour, minute, second, millisecond.
 
 Usage: 
 `{{ pm_utils.datediff('[datepart]', '[start_date_expression]', '[end_date_expression]') }}`