Editorial: Allow returning UNSET from DifferenceZonedDateTimeWithRoun…

…ding

When calling DifferenceZonedDateTimeWithRounding with smallestUnit and
largestUnit not the same unit, it's possible to hit the NudgeToZonedTime
path, where [[Total]] is UNSET. This should be reflected in the structured
header's return type.

Closes: #2879

spec/zoneddatetime.html

@@ -1394,7 +1394,7 @@ <h1>
           _roundingIncrement_: a positive integer,
           _smallestUnit_: a String,
           _roundingMode_: a String,
-        ): either a normal completion containing a Record with fields [[DurationRecord]] (a Duration Record) and [[Total]] (a mathematical value), or a throw completion
+        ): either a normal completion containing a Record with fields [[DurationRecord]] (a Duration Record) and [[Total]] (a mathematical value or ~unset~), or a throw completion
       </h1>
       <dl class="header">
         <dt>description</dt>