RFR: 8376271: ZipFile comment confusingly refers to "native" ZIP file implementation

[Eirik Bjørsnøs]

Please review this comment-only PR  which updates a field comment referring to `ZipFile.Source` as "native".

ZipFile has not used a native implementation since Java 8, so this comment may be confusing.

The comment seems to have been introduced in 2017 as part of JDK-8185582 when ZipFile was updated to use Cleaners instead of finalizers. (The use of quotation marks in "native" may have meant to indicate that this is "not really native", but I still find it confusing).

While visiting this comment, I also made an attempt to make the leading sentence of this field comment less mysterious. 

Comment only, trivial, low risk: `noreg-trivial`

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Commit messages:
 - Improve comment on the ZipFile.res field

Changes: https://git.openjdk.org/jdk/pull/29401/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376271
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/29401.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29401/head:pull/29401

PR: https://git.openjdk.org/jdk/pull/29401