[foreign-abi] RFR: Misc javadoc fixes

[Maurizio Cimadamore]

I did another pass on the ABI-related javadoc; this resulted in a biggie surgery of the javadoc in `CLinker` as well as
few minor changes/fixes in several places.

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Commit messages:
 - Misc javadoc fixes

Changes: https://git.openjdk.java.net/panama-foreign/pull/373/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=373&range=00
  Stats: 38 lines in 3 files changed: 3 ins; 2 del; 33 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/373.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/373/head:pull/373

PR: https://git.openjdk.java.net/panama-foreign/pull/373