[lworld] RFR: 8377171: [lworld] Comment warnings for internal valhalla annotations

[Chen Liang]

Add some warning comments for value-related annotations in jdk.internal, to discourage their misuse.

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Commit messages:
 - Warnings for value annotations

Changes: https://git.openjdk.org/valhalla/pull/2023/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2023&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377171
  Stats: 36 lines in 2 files changed: 18 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/valhalla/pull/2023.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2023/head:pull/2023

PR: https://git.openjdk.org/valhalla/pull/2023