[foreign-memaccess+abi] RFR: Update foreign linker doc

[Maurizio Cimadamore]

This patch refreshes the document describing the Foreign Linker API.

The biggest change is at the beginning, as we have to deal with the concept of zero-length memory segments (we had a section on memory addresses here).

Other than that, it's mostly cosmetic changes to rename `MemorySession` to `Arena`.

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Commit messages:
 - Fix more whitespaces
 - Fix whitespaces
 - Update foreign linker doc

Changes: https://git.openjdk.org/panama-foreign/pull/759/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=759&range=00
  Stats: 103 lines in 1 file changed: 7 ins; 8 del; 88 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/759.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/759/head:pull/759

PR: https://git.openjdk.org/panama-foreign/pull/759