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

[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

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Depends on: https://git.openjdk.org/jdk/pull/21431

Commit messages:
 - 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.

Changes: https://git.openjdk.org/jdk/pull/21949/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21949&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343752
  Stats: 71 lines in 3 files changed: 66 ins; 1 del; 4 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