[foreign-memaccess+abi] RFR: Improve documentation and factories of Arena [v3]

[Per Minborg]

On Tue, 8 Nov 2022 14:37:45 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> 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:
> 
>   Clarify throws WTE conditions

This PR is closed in favour of a PR against PR_20

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