[lworld] RFR: 8361307: [lworld] Clarify identity vs value in Class, Objects, and document limitations of value objects

[Roger Riggs]

Javadoc updates to clarify value class vs identity class distinctions when in preview-mode and not.

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Commit messages:
 - 8361307: [lworld] Clarify identity vs value in Class, Objects, and document limitations of value objects

Changes: https://git.openjdk.org/valhalla/pull/1513/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1513&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361307
  Stats: 286 lines in 10 files changed: 235 ins; 8 del; 43 mod
  Patch: https://git.openjdk.org/valhalla/pull/1513.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1513/head:pull/1513

PR: https://git.openjdk.org/valhalla/pull/1513