document nanoseconds duration limit of 104 days

docs/absolute.md

@@ -316,7 +316,8 @@ If you do need to calculate the difference between two `Temporal.Absolute`s in y
 For example, you might decide to base the calculation on your user's current time zone, or on UTC.
 
 Take care when using milliseconds, microseconds, or nanoseconds as the largest unit.
-For some dates, the resulting value may overflow `Number.MAX_SAFE_INTEGER`.
+For some durations, the resulting value may overflow `Number.MAX_SAFE_INTEGER` and will throw a `RangeError`.
+For example, `Number.MAX_SAFE_INTEGER` in nanoseconds is only 104.25 days.
 
 Example usage:
 ```js
@@ -333,8 +334,9 @@ missionLength.toLocaleString();
 epoch = new Temporal.Absolute(0n);
 billion = Temporal.Absolute.fromEpochSeconds(1e9);
 billion.difference(epoch);  // => PT1000000000S
-billion.difference(epoch, { largestUnit: 'hours' })  // => PT277777H46M40S
-billion.difference(epoch, { largestUnit: 'days' })  // => P11574DT1H46M40S
+billion.difference(epoch, { largestUnit: 'hours' });        // => PT277777H46M40S
+billion.difference(epoch, { largestUnit: 'days' });         // => P11574DT1H46M40S
+billion.difference(epoch, { largestUnit: 'nanoseconds' });  // => overflow; throws RangeError
 
 // Calculate the difference in years, eliminating the ambiguity by
 // explicitly using the corresponding calendar date in UTC:

docs/datetime.md

@@ -455,22 +455,27 @@ By default, the largest unit in the result is days.
 This is because months and years can be different lengths depending on which month is meant and whether the year is a leap year.
 
 Take care when using milliseconds, microseconds, or nanoseconds as the largest unit.
-For some dates, the resulting value may overflow `Number.MAX_SAFE_INTEGER`.
+For some durations, the resulting value may overflow `Number.MAX_SAFE_INTEGER` and will throw a `RangeError`.
+For example, `Number.MAX_SAFE_INTEGER` in nanoseconds is only 104.25 days.
 
 Usage example:
 ```javascript
 dt1 = Temporal.DateTime.from('1995-12-07T03:24:30.000003500');
 dt2 = Temporal.DateTime.from('2019-01-31T15:30');
-dt2.difference(dt1);                            // =>    P8456DT12H5M29.999996500S
-dt2.difference(dt1), { largestUnit: 'years' })  // => P23Y1M24DT12H5M29.999996500S
-dt1.difference(dt2), { largestUnit: 'years' })  // => throws RangeError
+dt2.difference(dt1);                                   // =>    P8456DT12H5M29.999996500S
+dt2.difference(dt1), { largestUnit: 'years' });        // => P23Y1M24DT12H5M29.999996500S
+dt2.difference(dt1), { largestUnit: 'nanoseconds' });  // => overflow; throws RangeError
+dt1.difference(dt2), { largestUnit: 'years' });        // => throws RangeError
 
 // Months and years can be different lengths
-[jan1, feb1, mar1] = [1, 2, 3].map(month => Temporal.DateTime.from({year: 2020, month, day: 1}));
-feb1.difference(jan1);                             // => P31D
-feb1.difference(jan1, { largestUnit: 'months' });  // => P1M
-mar1.difference(feb1);                             // => P29D
-mar1.difference(feb1, { largestUnit: 'months' });  // => P1M
+[jan1, feb1, mar1, apr1, may1] = 
+  [1, 2, 3, 4, 5].map(month => Temporal.DateTime.from({ year: 2020, month, day: 1 }));
+feb1.difference(jan1);                                   // => P31D
+feb1.difference(jan1, { largestUnit: 'months' });        // => P1M
+mar1.difference(feb1);                                   // => P29D
+mar1.difference(feb1, { largestUnit: 'months' });        // => P1M
+may1.difference(jan1);                                   // => P121D
+may1.difference(jan1), { largestUnit: 'nanoseconds' });  // => overflow; throws RangeError
 ```
 
 ### datetime.**equals**(_other_: Temporal.DateTime) : boolean