RFR: 8368953: Document the reason why Serial/Parallel/G1 use zero as dirty card value

[Thomas Schatzl]

Hi all,

  please review this trivial change to document the reason for the dirty/clean card value selection.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - 8368953

Changes: https://git.openjdk.org/jdk/pull/27575/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27575&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368953
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27575.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27575/head:pull/27575

PR: https://git.openjdk.org/jdk/pull/27575