[lworld] RFR: 8296711: [lworld] identity/value javadoc cleanup

[Roger Riggs]

Clarify the java doc descriptions in java.lang.ref classes to say that References can only be created to identity objects.

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Commit messages:
 - 8296711: [lworld] identity/value javadoc cleanup

Changes: https://git.openjdk.org/valhalla/pull/807/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=807&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296711
  Stats: 16 lines in 4 files changed: 5 ins; 6 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/807.diff
  Fetch: git fetch https://git.openjdk.org/valhalla pull/807/head:pull/807

PR: https://git.openjdk.org/valhalla/pull/807