RFR: JDK-8277074: Qualified exported types show up in JavaDoc

[Hannes Wallnöfer]

Please review a simple change to hide internal classes in generated documentation by adding doc comments containing a `@hidden` tag. I verified the fix by making sure `grep -r jdk.internal` returns no matches in the generated documentation tree.

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Commit messages:
 - JDK-8277074: Qualified exported types show up in JavaDoc

Changes: https://git.openjdk.org/jdk/pull/11163/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11163&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8277074
  Stats: 14 lines in 2 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11163.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11163/head:pull/11163

PR: https://git.openjdk.org/jdk/pull/11163