RFR: 8350147: Replace example in KEM class with the one from JEP 452 [v2]

[Bradford Wetmore]

> 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.

Bradford Wetmore has updated the pull request incrementally with one additional commit since the last revision:

  Codereview Comments from Weijun

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23652/files
  - new: https://git.openjdk.org/jdk/pull/23652/files/d0a9900e..70a3ae85

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23652&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23652&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 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