[jdk17] RFR: 8271489: (doc) Clarify Filter Factory example

[Roger Riggs]

Improve the clarity of comments in the ObjectInputFilter FilterInThread example.

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Commit messages:
 - 8271489: (doc) Clarify Filter Factory example

Changes: https://git.openjdk.java.net/jdk17/pull/293/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=293&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271489
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/293.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/293/head:pull/293

PR: https://git.openjdk.java.net/jdk17/pull/293