[foreign-memaccess+abi] RFR: Misc javadoc fixes

[Maurizio Cimadamore]

This short patch addresses some API review comments which have been raised by @jbf.

Following changes are made:

* addOnClose is renamed to addCloseAction, to remove ambiguity on when `add` happens
* some comments about allocation confinement and thread safety have been moved in the arenaAllocator factory (from the top level javadoc)
* The text in the thread-confinement section of `ResourceScope` has been massaged a little to describe how clients are expected to deal with issues when closing shared scopes in a more positive way (e.g. the new text suggests what to do "Add more synchronization" instead of saying "it's a bug"). The rewritten section also connects more nicely with what comes later (resource scope handles) which are all about avoiding close vs. access races.

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Commit messages:
 - Misc javadoc fixes

Changes: https://git.openjdk.java.net/panama-foreign/pull/515/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=515&range=00
  Stats: 28 lines in 6 files changed: 9 ins; 4 del; 15 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/515.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/515/head:pull/515

PR: https://git.openjdk.java.net/panama-foreign/pull/515