Date.is_leap_day seems wrong

[twopir]

utop [21]: Date.is_leap_year 2000;;
- : bool = true
utop [22]: Date.is_leap_day (Date.make 2000 2 29);;
- : bool = false

https://github.com/ocaml-community/calendar/blob/a447a88ae3c1e9873e32d2a95d3d3e7c5ed4a7da/src/date.ml#L212

This seems to just be defined incorrectly.

[twopir]

Or I could read the FAQ — sorry!

https://github.com/ocaml-community/calendar/blob/a447a88ae3c1e9873e32d2a95d3d3e7c5ed4a7da/calendarFAQ-2.6.txt#L663

[Drup]

The behavior may be correct, but it should be explained in the documentation of the function, not lost in the FAQ that most people don't know exist. :)