Editorial: mention "cf" in Note 1 of #sec-intl.numberformat-internal-slots

[FrankYFTang]

Mention "cf" in https://tc39.es/ecma402/#sec-intl.numberformat-internal-slots
see also https://unicode.org/reports/tr35/#Key_Type_Definitions Make it clear that "cf" is not supported via extension.

[sffc]

Seems good; @ryzokuken ?

[ryzokuken]

@sffc it does, but it made me think: cf and hc are two pieces of information that have been handled in the API using options, so maybe this list would get longer over time as the BCP 47 standardization process moves ahead. Maybe we need to make a note to add support for these new tags over time and even effectively deprecate the use of the options as we go about it (I'm not opinionated about the latter, but we should definitely talk about it, right?)

[sffc]

2021-07-01 discussion: https://github.com/tc39/ecma402/blob/master/meetings/notes-2021-07-01.md#editorial-mention-cf-in-note-1-of-sec-intlnumberformat-internal-slots-581

Follow-up for editors: #584

[ryzokuken]

Merging this since the relevant follow-up issues have been opened. Thanks @FrankYFTang!