Editorial: Distinguish "available time zone identifier" (#953)

Temporal will introduce separate definitions for "time zone identifier"
(well-formed) and "available time zone identifier" (known to an implementation)

Author: ptomato

spec/datetimeformat.html

@@ -1111,7 +1111,7 @@ <h1>Properties of Intl.DateTimeFormat Instances</h1>
       <li>[[Locale]] is a String value with the language tag of the locale whose localization is used for formatting.</li>
       <li>[[Calendar]] is a String value representing the <a href="https://unicode.org/reports/tr35/#UnicodeCalendarIdentifier">Unicode Calendar Identifier</a> used for formatting.</li>
       <li>[[NumberingSystem]] is a String value representing the <a href="https://unicode.org/reports/tr35/#UnicodeNumberSystemIdentifier">Unicode Number System Identifier</a> used for formatting.</li>
-      <li>[[TimeZone]] is a String value used for formatting that is either a time zone identifier from the IANA Time Zone Database or a UTC offset in ISO 8601 extended format.</li>
+      <li>[[TimeZone]] is a String value used for formatting that is either an available named time zone identifier or an offset time zone identifier.</li>
       <li>[[HourCycle]] is a String value indicating whether the 12-hour format (*"h11"*, *"h12"*) or the 24-hour format (*"h23"*, *"h24"*) should be used. *"h11"* and *"h23"* start with hour 0 and go up to 11 and 23 respectively. *"h12"* and *"h24"* start with hour 1 and go up to 12 and 24. [[HourCycle]] is only used when [[DateTimeFormat]] has an [[hour]] field.</li>
       <li>[[DateStyle]], [[TimeStyle]] are each either *undefined*, or a String value with values *"full"*, *"long"*, *"medium"*, or *"short"*.</li>
       <li>[[DateTimeFormat]] is a DateTime Format Record.</li>

spec/locales-currencies-tz.html

@@ -148,7 +148,7 @@ <h1>AvailableCanonicalCurrencies ( ): a List of Strings</h1>
     <h1>Use of the IANA Time Zone Database</h1>
 
     <p>
-      Implementations that adopt this specification must be time zone aware: they must use the IANA Time Zone Database <a href="https://www.iana.org/time-zones/">https://www.iana.org/time-zones/</a> to supply time zone identifiers and data used in ECMAScript calculations and formatting.
+      Implementations that adopt this specification must be time zone aware: they must use the IANA Time Zone Database <a href="https://www.iana.org/time-zones/">https://www.iana.org/time-zones/</a> to supply available named time zone identifiers and data used in ECMAScript calculations and formatting.
       This section defines how the IANA Time Zone Database should be used by time zone aware implementations.
       No String may be an available named time zone identifier unless it is a Zone name or a Link name in the IANA Time Zone Database.
       Available named time zone identifiers returned by ECMAScript built-in objects must use the casing found in the IANA Time Zone Database.
@@ -172,7 +172,7 @@ <h1>Use of the IANA Time Zone Database</h1>
 
     <emu-note>
       <p>
-        The IANA Time Zone Database offers build options that affect which time zone identifiers are primary.
+        The IANA Time Zone Database offers build options that affect which available named time zone identifiers are primary.
         The default build options merge different countries' time zones, for example *"Atlantic/Reykjavik"* is built as a Link to the Zone *"Africa/Abidjan"*.
         Geographically and politically distinct locations are likely to introduce divergent time zone rules in a future version of the IANA Time Zone Database.
         The exceptions above serve to mitigate these future-compatibility issues.