[foreign-abi] RFR: Fix javadoc issues in foreign-abi

[Maurizio Cimadamore]

This patch addresses several issues with foreign-abi javadoc, which are preventing javadoc to complete succesfully on
the jdk.incubator.foreign package.

I added the various bits of missing documentation and fixed all relevant warnings - the comments will likely need a
later pass, but at least now javadoc can be built w/o issues.

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Commit messages:
 - Fix javadoc issues

Changes: https://git.openjdk.java.net/panama-foreign/pull/230/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/230/webrev.00
  Stats: 81 lines in 4 files changed: 70 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/230.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/230/head:pull/230

PR: https://git.openjdk.java.net/panama-foreign/pull/230