[foreign-jextract] RFR: Update memory access and linker documents [v4]

[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:

 - Fix review comments
 - Fix section on SegmentAllocator (was containing forward-references to native method handles)

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/475/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/475/files/5850b37a..be46af2a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=475&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=475&range=02-03

  Stats: 27 lines in 1 file changed: 4 ins; 5 del; 18 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