RFR: 8358530: enhanced Properties#list javadoc

[cagliostro92]

Trivial PR to enhance Javadoc for the `Properties#list` method, which has cost me some debugging time.

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Commit messages:
 - 8358530: adds warnings against non-String values in Properties class documentation

Changes: https://git.openjdk.org/jdk/pull/25334/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25334&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358530
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/25334.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25334/head:pull/25334

PR: https://git.openjdk.org/jdk/pull/25334