RFR: JDK-8308913: Update core reflection for JEP 445 (preview)

[Joe Darcy]

Explain in java.lang.Class how unnamed classes are modeled in core reflection.

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Commit messages:
 - Cleanup and edit text.
 - Respond to review comments.
 - Appease jcheck.
 - JDK-8308913: Update core reflection for JEP 445 (preview)

Changes: https://git.openjdk.org/jdk/pull/14165/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14165&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308913
  Stats: 26 lines in 1 file changed: 25 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14165.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14165/head:pull/14165

PR: https://git.openjdk.org/jdk/pull/14165