<i18n dev> RFR: 8303275: Use {@Return and @linkplain in Locale and related classes

[Justin Lu]

This PR modifies the javadoc of methods in Locale, LocaleServiceProvider, and LocaleServiceProviderPool to use {@return and @linkplain.

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Commit messages:
 - fix conflicts
 - Locale copyright
 - Use @linkplain and {@return in Locale
 - Use {@return and spacing in LSPP
 - Use {@return and @linkplain in LSP

Changes: https://git.openjdk.org/jdk/pull/12780/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12780&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303275
  Stats: 107 lines in 3 files changed: 11 ins; 19 del; 77 mod
  Patch: https://git.openjdk.org/jdk/pull/12780.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12780/head:pull/12780

PR: https://git.openjdk.org/jdk/pull/12780