Document ECMA-402's prohibition on duplicate variant subtags, both in locales overall and in locales nested in transformed_extensions

[jswalden]

This fixes tc39/ecma402-mdn#18 which documents the change being made in tc39/ecma402#429.

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The existing text was unsuited to easily adding this restriction. I also found myself dissatisfied with the current wording, which piles up a bit too much prose in one paragraph, while not clearly delineating the possible locales values. I rewrote stuff to clearly explain how things all fit together.

I removed use of the term "Unicode BCP 47 locale identifier". If we're imposing a no-duplicate-variants requirement in transformed_extensions, beyond how TR35 presently defines it, the term is imprecise. And most readers of this document won't know the term anyway. It seems simplest to use "locale identifier" to refer to a string that matches the grammar and non-grammar restrictions, then "locale" refers to either a locale identifier string or to a Intl.Locale that wraps one.

The current text uses "tag" and "subtag" and "component" indiscriminately. The tag naming was fine when this page talked about language tags. It stopped making sense when this page changed to use "Unicode BCP 47 locale identifier". I changed the text to consistently use "subtag" and never "tag" for alphanumeric letter sequences, and I used "sequence" for any sequence of subtags.

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As to the actual change this PR concerns, it's debatable whether this needs to be documented. The new restriction is obscure; it's doubtful people are using transformed_extensions if no Intl functionality examines it. But the restriction isn't required in UTS35 or BCP 47 yet, so we really ought say it somewhere other than just the spec.

Back when this was all a wiki (so it goes), I would have just made these changes and gotten a post-hoc read-over. But I guess now everything gets a full review before it lands? I'll certainly rope in some ECMA-402 people, but having a non-domain expert look at this seems pretty desirable given this page is targeted at web developers.

[sideshowbarker]

Minor nits about punctuation (using a colon to introduce a bulleted list), and two spaces after full stops (MDN style is to use only one space after a full stop).

[jswalden]

Okay, I folded all the suggestions into whichever of the two revs was appropriate, and I updated the branch accordingly. My git-fu for editing sequences of patches, and squashing/reordering/etc. them, is not great, but I think I got it all right. Could be worth a quick once-over if you're paranoid. Otherwise, this should be good to go.

[sideshowbarker]

Great to see you here and great to see this refinement going in. Congrats on landing your first docs patch under the new system — welcome aboard 🎉