<i18n dev> RFR: 8378723: Locale variant delimiter is unclear

[Justin Lu]

Locale APIs which accept BCP 47 tags use hyphen for multiple variant subtags. Locale APIs that accept multiple (non BCP 47 subtag) variants use underscores. For example,

`Locale.of` supports multiple variants separated by underscore.
`Locale.forLanguageTag` supports multiple variants separated by hyphen.
`Locale.Builder.setLanguageTag` supports multiple variants separated by hyphen.
`Locale.Builder.setVariant` supports multiple variants separated by either underscore or hyphen. 

The current wording is not detailed enough and implies both formats are interchangeable within `Locale`. This PR updates the wording to make this more apparent. Note that I left the constructor behavior out in the new wording as to not call attention to it.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/29970/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29970&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378723
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29970.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29970/head:pull/29970

PR: https://git.openjdk.org/jdk/pull/29970