Editorial: Document that DefaultLocale() returns must not include a "…

…-u-" extension

This is already required:
* LookupMatcher can return { [[locale]]: DefaultLocale(), ... }.
* ResolveLocale uses that result's [[locale]] as the name of a field in
  its _localeData_ argument and asserts that the corresponding value is
  a Record.
* The _localeData_ argument to ResolveLocale is generally provided from
  the value of a service constructor's [[LocaleData]] slot.
* "Internal slots of Service Constructors" constrains those slots to be
  Records "that have fields for each locale contained in
  [[AvailableLocales]]", and constrains their [[AvailableLocales]] slots
  to include only language tags that "must not have a Unicode locale
  extension sequence".

Therefore, if DefaultLocale() *were* to return a value with a Unicode
locale extension sequence, use of it in ResolveLocale would fail to find
the required Record at a field with that name in e.g. the service
constructor's [[LocaleData]].

spec/locales-currencies-tz.html

@@ -118,7 +118,7 @@ <h1>
       </h1>
       <dl class="header">
         <dt>description</dt>
-        <dd>The returned String value represents the structurally valid (<emu-xref href="#sec-isstructurallyvalidlanguagetag"></emu-xref>) and canonicalized (<emu-xref href="#sec-canonicalizeunicodelocaleid"></emu-xref>) language tag for the host environment's current locale.</dd>
+        <dd>The returned String value represents the structurally valid (<emu-xref href="#sec-isstructurallyvalidlanguagetag"></emu-xref>) and canonicalized (<emu-xref href="#sec-canonicalizeunicodelocaleid"></emu-xref>) language tag for the host environment's current locale. It must not contain a Unicode locale extension sequence.</dd>
       </dl>
     </emu-clause>
   </emu-clause>