[jdk20] RFR: 8301864: ObjectInputFilter example incorrectly calls rejectUndecideClass

[Roger Riggs]

The example code in ObjectInputFilter for the FilterInThread filter factory does not do what is intended and is poorly described. Clarifies that the JVM-wide filter and the thread local filter are merged and will reject classes that are otherwise not accepted or rejected. If a stream specific filter is set and does not accept or reject a class, the combined filter is applied.

This is a doc-only change.

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Commit messages:
 - 8301864: ObjectInputFilter example incorrectly calls rejectUndecidedClass

Changes: https://git.openjdk.org/jdk20/pull/121/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=121&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301864
  Stats: 19 lines in 1 file changed: 4 ins; 9 del; 6 mod
  Patch: https://git.openjdk.org/jdk20/pull/121.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/121/head:pull/121

PR: https://git.openjdk.org/jdk20/pull/121