RFR: 8366193: Add comments about ResolvedFieldEntry::copy_from()

[Ioi Lam]

[JDK-8293980](https://bugs.openjdk.org/browse/JDK-8293980) added `ResolvedFieldEntry::copy_from()` to prevent random bits in the CDS archive. However, I forgot to add a comment to explain why this is necessary.

This PR adds the missing comment, which will be useful if we consider alternative solutions (using `memset()` in constructor??) in the future.

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Commit messages:
 - Fixed whitespace
 - 8366193: Add comments about ResolvedFieldEntry::copy_from()

Changes: https://git.openjdk.org/jdk/pull/26946/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26946&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366193
  Stats: 36 lines in 3 files changed: 33 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26946.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26946/head:pull/26946

PR: https://git.openjdk.org/jdk/pull/26946