RFR: 8327997: G1: Move G1ScanClosureBase::reference_iteration_mode to subclass

[Albert Mingkun Yang]

Simple moving a method from super class to sub class and some documentation.

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Commit messages:
 - g1-card-scan-mode

Changes: https://git.openjdk.org/jdk/pull/18244/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18244&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327997
  Stats: 12 lines in 1 file changed: 10 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18244.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18244/head:pull/18244

PR: https://git.openjdk.org/jdk/pull/18244