RFR: 8265684: implement Sealed Classes as a standard feature in Java, javadoc changes

[Vicente Romero]

Please review the javadoc related changes to make sealed classes a final feature in Java. The patch is mostly removing preview features related code that is not necessary anymore in javadoc.

TIA

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Commit messages:
 - 8265684: implement Sealed Classes as a standard feature in Java, javadoc changes

Changes: https://git.openjdk.java.net/jdk/pull/3613/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3613&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265684
  Stats: 253 lines in 18 files changed: 0 ins; 191 del; 62 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3613.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3613/head:pull/3613

PR: https://git.openjdk.java.net/jdk/pull/3613