RFR: 8350147: Replace example in KEM class with the one from JEP 452

Sat Feb 15 05:07:37 UTC 2025

The example code in the JEP was much clearer than that in the current `KEM` class.  It's easier to keep the various nested classes straight, rather than using `var`s which require scrolling to see what the return types are. 

Added/tweaked example for clarity.

Generated/checked javadocs, but no regression/JCK tests is necessary.

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

Commit messages:
 - 8350147: Replace example in KEM class with the one from JEP 452

Changes: https://git.openjdk.org/jdk/pull/23652/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23652&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350147
  Stats: 32 lines in 1 file changed: 18 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/23652.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23652/head:pull/23652

PR: https://git.openjdk.org/jdk/pull/23652