[foreign-memaccess+abi] RFR: Update foreign linker doc [v3]

[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 refreshed the contents of this pull request, and previous commits have been removed. Incremental views are not available. The pull request now contains four commits:

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

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/759/files
  - new: https://git.openjdk.org/panama-foreign/pull/759/files/dd4bfb80..c25a2d5c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=759&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=759&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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