RFR: 8343752: The javadoc should contain a note about usages of requires transitive java.base; [v3]

[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 incrementally with one additional commit since the last revision:

  Using <br> instead of <div>, as suggested.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21949/files
  - new: https://git.openjdk.org/jdk/pull/21949/files/8b8d8296..22cb35b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21949&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21949&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 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