[foreign-memaccess+abi] RFR: Update foreign linker doc [v2]

[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`.

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

 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/759/files
  - new: https://git.openjdk.org/panama-foreign/pull/759/files/91d38893..dd4bfb80

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=759&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=759&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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