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

[Per Minborg]

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

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Commit messages:
 - Fix typo
 - Specify native memory
 - Update arena doc

Changes: https://git.openjdk.org/panama-foreign/pull/752/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=752&range=00
  Stats: 72 lines in 2 files changed: 39 ins; 18 del; 15 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