[Editorial] Fix Intl.PluralRules after the merging of PR 811

Split the editorial fix from tc39#811

1. Add text to explain ComputedRoundingPriority in section "15.4 Properties of Intl.NumberFormat Instances"
https://tc39.es/ecma402/#sec-properties-of-intl-numberformat-instances and
"16.4 Properties of Intl.PluralRules Instances
https://tc39.es/ecma402/#sec-properties-of-intl-pluralrules-instances

2. Add ComputedRoundingPriority to Intl.PluralRules https://tc39.es/ecma402/#sec-intl.pluralrules
Without this, the spec is not implementable because InitializePluralRules call "SetNumberFormatDigitOptions(pluralRules, options, +0𝔽, 3𝔽, "standard")." and SetNumberFormatDigitOptions assume the intlObj has [[ComputedRoundingPriority]]

spec/numberformat.html

@@ -17,7 +17,7 @@ <h1>Intl.NumberFormat ( [ _locales_ [ , _options_ ] ] )</h1>
 
       <emu-alg>
         1. If NewTarget is *undefined*, let _newTarget_ be the active function object, else let _newTarget_ be NewTarget.
-        1. Let _numberFormat_ be ? OrdinaryCreateFromConstructor(_newTarget_, *"%NumberFormat.prototype%"*, &laquo; [[InitializedNumberFormat]], [[Locale]], [[DataLocale]], [[NumberingSystem]], [[Style]], [[Unit]], [[UnitDisplay]], [[Currency]], [[CurrencyDisplay]], [[CurrencySign]], [[MinimumIntegerDigits]], [[MinimumFractionDigits]], [[MaximumFractionDigits]], [[MinimumSignificantDigits]], [[MaximumSignificantDigits]], [[RoundingType]], [[Notation]], [[CompactDisplay]], [[UseGrouping]], [[SignDisplay]], [[RoundingMode]], [[RoundingIncrement]], [[ComputedRoundingPriority]], [[TrailingZeroDisplay]], [[BoundFormat]] &raquo;).
+        1. Let _numberFormat_ be ? OrdinaryCreateFromConstructor(_newTarget_, *"%NumberFormat.prototype%"*, &laquo; [[InitializedNumberFormat]], [[Locale]], [[DataLocale]], [[NumberingSystem]], [[Style]], [[Unit]], [[UnitDisplay]], [[Currency]], [[CurrencyDisplay]], [[CurrencySign]], [[MinimumIntegerDigits]], [[MinimumFractionDigits]], [[MaximumFractionDigits]], [[MinimumSignificantDigits]], [[MaximumSignificantDigits]], [[RoundingType]], [[Notation]], [[CompactDisplay]], [[UseGrouping]], [[SignDisplay]], [[RoundingIncrement]], [[RoundingMode]], [[ComputedRoundingPriority]], [[TrailingZeroDisplay]], [[BoundFormat]] &raquo;).
         1. Perform ? InitializeNumberFormat(_numberFormat_, _locales_, _options_).
         1. If the implementation supports the normative optional constructor mode of <emu-xref href="#legacy-constructor"></emu-xref>, then
           1. Let _this_ be the *this* value.
@@ -547,8 +547,9 @@ <h1>Properties of Intl.NumberFormat Instances</h1>
         [[SignDisplay]] is one of the String values *"auto"*, *"always"*, *"never"*, *"exceptZero"*, or *"negative"*, specifying when to include a sign (with non-*"auto"* options respectively corresponding with inclusion always, never, only for non-zero numbers, or only for non-zero negative numbers).
         In scientific notation, this slot affects the sign display of the mantissa but not the exponent.
       </li>
-      <li>[[RoundingMode]] is one of the String values in the Identifier column of <emu-xref href="#table-intl-rounding-modes"></emu-xref>, specifying which rounding mode to use.</li>
       <li>[[RoundingIncrement]] is an integer-valued Number that evenly divides 10, 100, 1000, or 10000 into tenths, fifths, quarters, or halves. It indicates the increment at which rounding should take place relative to the calculated rounding magnitude. For example, if [[MaximumFractionDigits]] is 2 and [[RoundingIncrement]] is 5, then the number is rounded to the nearest 0.05 ("nickel rounding").</li>
+      <li>[[RoundingMode]] is one of the String values in the Identifier column of <emu-xref href="#table-intl-rounding-modes"></emu-xref>, specifying which rounding mode to use.</li>
+      <li>[[ComputedRoundingPriority]] is one of the String values *"auto"*, *"lessPrecision"*, or *"morePrecision"*, specifying the strategy to resolve mixed fraction digits and significant digits settings.</li>
       <li>[[TrailingZeroDisplay]] is one of the String values *"auto"* or *"stripIfInteger"*, indicating whether to strip trailing zeros if the formatted number is an integer (i.e., has no non-zero fraction digit).</li>
     </ul>
 

spec/pluralrules.html

@@ -17,7 +17,7 @@ <h1>Intl.PluralRules ( [ _locales_ [ , _options_ ] ] )</h1>
 
       <emu-alg>
         1. If NewTarget is *undefined*, throw a *TypeError* exception.
-        1. Let _pluralRules_ be ? OrdinaryCreateFromConstructor(NewTarget, *"%PluralRules.prototype%"*, &laquo; [[InitializedPluralRules]], [[Locale]], [[Type]], [[MinimumIntegerDigits]], [[MinimumFractionDigits]], [[MaximumFractionDigits]], [[MinimumSignificantDigits]], [[MaximumSignificantDigits]], [[RoundingType]], [[RoundingMode]], [[RoundingIncrement]], [[TrailingZeroDisplay]] &raquo;).
+        1. Let _pluralRules_ be ? OrdinaryCreateFromConstructor(NewTarget, *"%PluralRules.prototype%"*, &laquo; [[InitializedPluralRules]], [[Locale]], [[Type]], [[MinimumIntegerDigits]], [[MinimumFractionDigits]], [[MaximumFractionDigits]], [[MinimumSignificantDigits]], [[MaximumSignificantDigits]], [[RoundingType]], [[RoundingIncrement]], [[RoundingMode]], [[ComputedRoundingPriority]], [[TrailingZeroDisplay]] &raquo;).
         1. Return ? InitializePluralRules(_pluralRules_, _locales_, _options_).
       </emu-alg>
     </emu-clause>
@@ -263,9 +263,10 @@ <h1>Properties of Intl.PluralRules Instances</h1>
       <li>[[MinimumIntegerDigits]] is a non-negative integer Number value indicating the minimum integer digits to be used.</li>
       <li>[[MinimumFractionDigits]] and [[MaximumFractionDigits]] are non-negative integer Number values indicating the minimum and maximum fraction digits to be used. Numbers will be rounded or padded with trailing zeroes if necessary.</li>
       <li>[[MinimumSignificantDigits]] and [[MaximumSignificantDigits]] are positive integer Number values indicating the minimum and maximum fraction digits to be used. Either none or both of these properties are present; if they are, they override minimum and maximum integer and fraction digits.</li>
-      <li>[[RoundingType]] is one of the values ~fractionDigits~, ~significantDigits~, ~morePrecision~, or ~lessPrecision~, indicating which rounding strategy to use, as discussed in <emu-xref href="#sec-properties-of-intl-numberformat-instances"></emu-xref>.</li>
-      <li>[[RoundingMode]] is one of the String values in the Identifier column of <emu-xref href="#table-intl-rounding-modes"></emu-xref>, specifying which rounding mode to use.</li>
       <li>[[RoundingIncrement]] is an integer-valued Number that evenly divides 10, 100, 1000, or 10000 into tenths, fifths, quarters, or halves. It indicates the increment at which rounding should take place relative to the calculated rounding magnitude. For example, if [[MaximumFractionDigits]] is 2 and [[RoundingIncrement]] is 5, then the number is rounded to the nearest 0.05 ("nickel rounding").</li>
+      <li>[[RoundingMode]] is one of the String values in the Identifier column of <emu-xref href="#table-intl-rounding-modes"></emu-xref>, specifying which rounding mode to use.</li>
+      <li>[[ComputedRoundingPriority]] is one of the String values *"auto"*, *"lessPrecision"*, or *"morePrecision"*, specifying the strategy to resolve mixed fraction digits and significant digits settings.</li>
+      <li>[[RoundingType]] is one of the values ~fractionDigits~, ~significantDigits~, ~morePrecision~, or ~lessPrecision~, indicating which rounding strategy to use, as discussed in <emu-xref href="#sec-properties-of-intl-numberformat-instances"></emu-xref>.</li>
       <li>[[TrailingZeroDisplay]] is one of the String values *"auto"* or *"stripIfInteger"*, indicating whether to strip trailing zeros if the formatted number is an integer (i.e., has no non-zero fraction digit).</li>
     </ul>
   </emu-clause>