[foreign-jextract] RFR: Update memory access and linker documents [v3]

[Maurizio Cimadamore]

> This patch updates the two documents describing usage of the foreign memory access and foreign linker API.

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

 - Another ffi fix
 - Minor adjustments to the ffi document

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/475/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/475/files/63692c09..5850b37a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=475&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=475&range=01-02

  Stats: 7 lines in 2 files changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/475.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/475/head:pull/475

PR: https://git.openjdk.java.net/panama-foreign/pull/475