[DOCS] Add info on calendar v fixed interval. Closes elastic#29410

docs/reference/aggregations/bucket/datehistogram-aggregation.asciidoc

@@ -343,6 +343,16 @@ second document falls into the bucket for 1 October 2015:
 <1> The `key_as_string` value represents midnight on each day
     in the specified timezone.
 
+WARNING: When using time zones that follow DST (daylight savings time) changes,
+buckets close to the moment when those changes happen can have slightly different
+sizes than you would expect from the used `interval`.
+For example, consider a DST start in the `CET` time zone: on 27 March 2016 at 2am,
+clocks were turned forward 1 hour to 3am local time. If you use `day` as `interval`,
+the bucket covering that day will only hold data for 23 hours instead of the usual
+24 hours for other buckets. The same is true for shorter intervals, like 12h,
+where you'll have only a 11h bucket on the morning of 27 March when the DST shift
+happens.
+
 ===== Offset
 
 Use the `offset` parameter to change the start value of each bucket by the