Document duration arithmetic behaviour (#918)

## What is the goal of this PR?

We document the edge case behaviour of datetime-tz arithmetic with
durations as implemented in typedb/typedb#7329.

Author: dmitrii-ubskii

typeql/modules/ROOT/pages/values/duration.adoc

@@ -87,6 +87,34 @@ When used with a value of type datetime-tz, arithmetic operations with duration
 2024-03-30T12:00:00 Europe/London + PT24H == 2024-03-31T13:00:00 
 ----
 
+If adding months and days results in an ambiguous datetime in the time zone, the earlier datetime is used.
+
+[,typeql]
+----
+# London DST change occurred on 2024-10-27 02:00:00 BST
+# I.e., on 2024-10-27, the time span between 01:00:00 and 02:00:00 occurs twice
+2024-10-26T01:30:00 Europe/London + P1D == 2024-10-27T01:30:00 BST
+
+# Between these two local datetimes, the earliest is BST
+2024-10-26T01:30:00 Europe/London + PT24H == 2024-10-27T01:30:00 BST
+2024-10-26T01:30:00 Europe/London + PT25H == 2024-10-27T01:30:00 GMT
+----
+
+If adding months and days results in a datetime that does not exist in the time zone, i.e. if there is a gap in the time zone, the datetime
+is increased by the length of the gap.
+
+[,typeql]
+----
+# London DST change occurred on 2024-03-31 01:00:00 GMT
+# I.e., there is a gap on 2024-03-31 between 01:00:00 and 02:00:00
+2024-03-30T01:30:00 Europe/London + P1D == 2024-03-31T02:30:00 BST
+
+# This is the same as adding 24 hours
+2024-03-30T01:30:00 Europe/London + PT24H == 2024-03-31T02:30:00 BST
+----
+
+The nanosecond part of the duration is added directly to the underlying UTC timestamp, and as such requires no special handling.
+
 === Defining a duration valued attribute
 
 [,typeql]