Docs: Fix documentation on ZDT add/subtract order of operations

The documentation incorrectly stated that the order of operations for
subtraction is in reverse. That is not what we do and also not what
RFC 5545 says. The correct thing is to always do the calendar math first,
then the exact time math.

Fix the text, and also deduplicate it from subtract(), replacing it with
a reference to add().

Closes: #2358

docs/zoneddatetime.md

@@ -882,11 +882,9 @@ The `duration` argument is an object with properties denoting a duration, such a
 
 Addition and subtraction are performed according to rules defined in [RFC 5545 (iCalendar)](https://tools.ietf.org/html/rfc5545):
 
-- Add/subtract the date portion of a duration using calendar days, like (like `Temporal.PlainDateTime`).
+- Add/subtract the date portion of a duration using calendar arithmetic (like `Temporal.PlainDateTime`).
   The result will automatically adjust for Daylight Saving Time using the rules of this instance's `timeZone` field.
-- Add/subtract the time portion of a duration using real-world time, like (like `Temporal.Instant`).
-- Addition (or subtraction of a negative duration) is performed in order from largest unit to smallest unit.
-- Subtraction (or addition of a negative duration) is performed in order from smallest unit to largest unit.
+- Add/subtract the time portion of a duration using real-world time (like `Temporal.Instant`).
 - If a result is ambiguous or invalid due to a time zone offset transition, the later of the two possible instants will be used for time-skipped transitions and the earlier of the two possible instants will be used for time-repeated transitions.
   This behavior corresponds to the default `disambiguation: 'compatible'` option used in `from` and used by legacy `Date` and moment.js.
 
@@ -945,24 +943,7 @@ This method subtracts a `duration` from `zonedDateTime`.
 
 The `duration` argument is an object with properties denoting a duration, such as `{ hours: 5, minutes: 30 }`, or a `Temporal.Duration` object. Subtracting a negative duration like `{ hours: -5, minutes: -30 }` is equivalent to adding the absolute value of that duration.
 
-Addition and subtraction are performed according to rules defined in [RFC 5545 (iCalendar)](https://tools.ietf.org/html/rfc5545):
-
-- Add/subtract the date portion of a duration using calendar days, like (like `Temporal.PlainDateTime`).
-  The result will automatically adjust for Daylight Saving Time using the rules of this instance's `timeZone` field.
-- Add/subtract the time portion of a duration using real-world time, like (like `Temporal.Instant`).
-- Addition (or subtraction of a negative duration) is performed in order from largest unit to smallest unit.
-- Subtraction (or addition of a negative duration) is performed in order from smallest unit to largest unit.
-- If a result is ambiguous or invalid due to a time zone offset transition, the later of the two possible instants will be used for time-skipped transitions and the earlier of the two possible instants will be used for time-repeated transitions.
-  This behavior corresponds to the default `disambiguation: 'compatible'` option used in `from` and used by legacy `Date` and moment.js.
-
-These rules make arithmetic with `Temporal.ZonedDateTime` "DST-safe", which means that the results most closely match the expectations of both real-world users and implementers of other standards-compliant calendar applications. These expectations include:
-
-- Adding or subtracting days should keep clock time consistent across DST transitions.
-  For example, if you have an appointment on Saturday at 1:00PM and you ask to reschedule it 1 day later, you would expect the reschedule appointment to still be at 1:00PM, even if there was a DST transition overnight.
-- Adding or subtracting the time portion of a duration should ignore DST transitions.
-  For example, a friend you've asked to meet in in 2 hours will be annoyed if you show up 1 hour or 3 hours later.
-- There should be a consistent and relatively-unsurprising order of operations.
-- If results are at or near a DST transition, ambiguities should be handled automatically (no crashing) and deterministically.
+Addition and subtraction are performed according to rules defined in [RFC 5545 (iCalendar)](https://tools.ietf.org/html/rfc5545), as described above in `add()`.
 
 Some arithmetic operations may be ambiguous, e.g. because months have different lengths.
 For example, subtracting one month from October 31 would result in September 31, which doesn't exist.