[m-rev.] for review: extend documentation of calendar/same_date/2

[Julien Fischer]

For review by anyone.

Extend the documentation of calendar.same_date/2.

library/calender.m:
    In particular, add a reminder that the reuslt are only meaningful if the
    two arguments are date_times in the same time zone.

Julien.

diff --git a/library/calendar.m b/library/calendar.m
index 05128b5d0..7d91e24e4 100644
--- a/library/calendar.m
+++ b/library/calendar.m
@@ -277,7 +277,10 @@
     % same_date(A, B):
     %
     % Succeed if-and-only-if A and B refer to the exact same day.
-    % Their time components are ignored.
+    % Their time components are ignored. A and B should refer to
+    % the same time zone; comparing dates in different time zones
+    % may give incorrect results because the same point in time can
+    % fall on different days in different time zones.
     %
 :- pred same_date(date_time::in, date_time::in) is semidet.