RFR: 8328679: Improve comment for UNSAFE_ENTRY_SCOPED in unsafe.cpp [v3]

[Maurizio Cimadamore]

> This small PR is an attempt at improving the comment in the UNSAFE_ENTRY_SCOPED macro.
> I found that the comment contained some duplicated claims, did not properly introduced the concepts it referred to, and some of its claims were not explained in full.
> 
> As this is a delicate part of how FFM ensures the "no use after free" guarantee, I think it would be a good idea for this comment to better reflect what the expectations are.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18429/files
  - new: https://git.openjdk.org/jdk/pull/18429/files/1f1c0684..35246ded

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18429&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18429&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18429.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18429/head:pull/18429

PR: https://git.openjdk.org/jdk/pull/18429