RFR: 8308645: Javadoc of FFM API needs to be refreshed

Mon Jun 12 11:08:02 UTC 2023

This is the same PR as https://github.com/openjdk/jdk/pull/14098, but backported to the JDK 21 repo fork.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk21/pull/7/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=7&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308645
  Stats: 964 lines in 28 files changed: 357 ins; 138 del; 469 mod
  Patch: https://git.openjdk.org/jdk21/pull/7.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/7/head:pull/7

PR: https://git.openjdk.org/jdk21/pull/7