RFR: 8343752: The javadoc should contain a note about usages of requires transitive java.base; [v2]

[Jan Lahoda]

> When https://github.com/openjdk/jdk/pull/21431 is integrated, there will be a new preview language feature: `requires transitive java.base;`. It would be good to show a warning box in the javadoc about the use of the feature. This PR is attempting to add such a warning box.
> 
> Given it is likely this feature will either graduate or be dropped in the span of a few releases, I tried to make the code (relatively) easy to remove: it should be enough to delete the `if (javaBase != null && indirectPackages.keySet().contains(javaBase)) {` and the then section, keeping the else section + some slight cleanup.
> 
> The new javadoc can be seen here:
> https://cr.openjdk.org/~jlahoda/8343752/updated/api/java.se/module-summary.html#preview-requires-transitive-java.base

Jan Lahoda has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Adding forgotten file.
 - Merge branch 'JDK-8335989' into preview-javadoc-requires-transitive-java.base
 - Better detection of requires transitive java.base; - fixes tests.
 - Cleanup: avoid unnecessary bundle key.
 - Adding a test, adjusting the message.
 - Improving the UI, as suggested.
 - Creating a separate javadoc section for the preview 'requires transitive java.base;' in the java.se module.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21949/files
  - new: https://git.openjdk.org/jdk/pull/21949/files/0bebf867..8b8d8296

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21949&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21949&range=00-01

  Stats: 299476 lines in 1988 files changed: 125495 ins; 150738 del; 23243 mod
  Patch: https://git.openjdk.org/jdk/pull/21949.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21949/head:pull/21949

PR: https://git.openjdk.org/jdk/pull/21949