Document PR #429

[sffc]

Document tc39/ecma402#429 or decide not to document it.

@jswalden @FrankYFTang

[jswalden]

I have (I think) most of this change written locally, just need to verify my changes work in the new world of MDN being roughly Not A Wiki any more.

[jswalden]

MDN changes landed, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl now mentions that variant subtags must be unique, including within a "t" extension. Anyone who wants to should feel free to give the changes in the PR a read-through!

[sffc]

Nice! Thank you! Let's close this issue then and update the progress tracking spreadsheet.