[foreign-memaccess+abi] RFR: Improve documentation and factories of Arena [v2]

[Per Minborg]

> This PR proposes some improvements in the documentation of the new Arena interface as well as moving out functionality to internal classes.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Add thread-safety spec

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/752/files
  - new: https://git.openjdk.org/panama-foreign/pull/752/files/ccc9d9f3..8d03e541

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=752&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=752&range=00-01

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/752.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/752/head:pull/752

PR: https://git.openjdk.org/panama-foreign/pull/752