[foreign-memaccess+abi] RFR: Update linker docs to reflect valist removal

[Maurizio Cimadamore]

Small update to foreign linker document to remove section on VaList.

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Commit messages:
 - Update footnotes
 - Update ffi doc to reflect valist changes

Changes: https://git.openjdk.org/panama-foreign/pull/764/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=764&range=00
  Stats: 751 lines in 2 files changed: 695 ins; 51 del; 5 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/764.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/764/head:pull/764

PR: https://git.openjdk.org/panama-foreign/pull/764