RFR: 8339781: Better use of Javadoc tags in javax.lang.model

[Joe Darcy]

More extensive use of definition HTML tag and index javadoc tag.

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Commit messages:
 - Update copyright.
 - Fix typos and expand scope.
 - Merge branch 'master' into JDK-8339781
 - JDK-8339781: Better use of Javadoc tags in javax.lang.model

Changes: https://git.openjdk.org/jdk/pull/20917/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20917&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339781
  Stats: 20 lines in 4 files changed: 1 ins; 0 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/20917.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20917/head:pull/20917

PR: https://git.openjdk.org/jdk/pull/20917