document potentially-surprising Month <-> Enum (#8923)

Based on a [forum question] by Alxander Bernauer.

[forum question]: https://discuss.daml.com/t/enum-instance-for-month-is-misleading/2098?u=gary_verhaegen

CHANGELOG_BEGIN
CHANGELOG_END

compiler/damlc/daml-stdlib-src/DA/Date/Types.daml

@@ -17,6 +17,9 @@ data DayOfWeek
     | Sunday
 
 -- | The `Month` type represents a month in the Gregorian calendar.
+--
+-- Note that, while `Month` has an `Enum` instance, the `toEnum` and `fromEnum`
+-- functions start counting at 0, i.e. `toEnum 1 :: Month` is `Feb`.
 data Month
     = Jan
     | Feb