RFR: 8308281: Java snippets in the FFM API need to be updated [v6]

[Per Minborg]

> As the API has improved over the recent releases, not all `{@snippet ..}` sections have been kept in sync.
> 
> This PR suggests all snippets used should be verified against real code that is placed in a new `snippet-files` folder and erroneous snippets are updated.
> 
> In this PR, it is suggested duplicating code in the `Snippets.java` class and in the JavaDocs. The benefit of this is that code is directly visible in the code and not only in the generated javadoc.
> 
> Another thing to think about is if there should be on single `Snippets.java` class or separate ones for each FFM class.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove last hybrid snippet

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14030/files
  - new: https://git.openjdk.org/jdk/pull/14030/files/cb114ce5..06d04b24

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14030&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14030&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14030.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14030/head:pull/14030

PR: https://git.openjdk.org/jdk/pull/14030