RFR: JDK-8324342: doclet should default @since for a nested class to that of its enclosing class

[Jonathan Gibbons]

Please review changes to the support for `since`, `author`, and `version` tags, such that if there are no such tags on a nested class, the doclet will look for any such tags in enclosing classes.

-------------

Commit messages:
 - JDK-8324342: doclet should default @since for a nested class to that of its enclosing class

Changes: https://git.openjdk.org/jdk/pull/18407/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324342
  Stats: 189 lines in 6 files changed: 155 ins; 4 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/18407.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18407/head:pull/18407

PR: https://git.openjdk.org/jdk/pull/18407