RFR: Minor cleanup, preview and since

[Chen Liang]

Add a preview feature for the NR jep, other stylistic cleanup

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

Commit messages:
 - Minor cleanup, preview and since

Changes: https://git.openjdk.org/valhalla/pull/1927/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1927&range=00
  Stats: 15 lines in 4 files changed: 8 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/1927.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1927/head:pull/1927

PR: https://git.openjdk.org/valhalla/pull/1927