RFR: 8338333: Add jls links to javax.lang.model.element.Modifier

[Joe Darcy]

The javadoc in javax.lang.model.element.Modifier would be more informative to readers if it linked to the relevant JLS sections discussing that modifier.

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Commit messages:
 - JDK-8338333: Add jls links to javax.lang.model.element.Modifier

Changes: https://git.openjdk.org/jdk/pull/20572/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20572&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338333
  Stats: 92 lines in 1 file changed: 81 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/20572.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20572/head:pull/20572

PR: https://git.openjdk.org/jdk/pull/20572