[foreign-abi] RFR: Fix javadoc issues in foreign-abi [v2]

[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.

Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:

 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/FunctionDescriptor.java
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/CSupport.java
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/230/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/230/files/975bda3f..bfd0a3b9

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/230/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/230/webrev.00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 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