<i18n dev> RFR: 8318466: Improve spec of NumberFormat's methods with unsupported operations

[Justin Lu]

Please review this simple change which refines the specification of some NumberFormat methods (with unsupported operations) to separate the API and implementation specification.

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

Commit messages:
 - cleanup setRoundingMode()
 - Merge branch 'master' into JDK-8318466-NumberFormat-Unsupported
 - reflect review
 - init

Changes: https://git.openjdk.org/jdk/pull/16464/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16464&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318466
  Stats: 36 lines in 1 file changed: 9 ins; 11 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/16464.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16464/head:pull/16464

PR: https://git.openjdk.org/jdk/pull/16464