[foreign-jextract] RFR: Refresh docs to reflect Java 18 API changes

[Maurizio Cimadamore]

This patch updates the memory access and linker docs to reflect the Java 18 API changes.

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

Commit messages:
 - Refresh docs to reflect Java 18 API changes

Changes: https://git.openjdk.java.net/panama-foreign/pull/630/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=630&range=00
  Stats: 100 lines in 2 files changed: 12 ins; 7 del; 81 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/630.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/630/head:pull/630

PR: https://git.openjdk.java.net/panama-foreign/pull/630