RFR: 8378723: Locale variant delimiter is unclear [v2]

[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.

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Describe basis for underscore support

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/29970/files
  - new: https://git.openjdk.org/jdk/pull/29970/files/bae0ef72..e161204b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29970&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29970&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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