RFR: 8367450: Shenandoah: Log the composition of the collection set [v6]

[William Kemper]

> Reuse the same log message from GenShen which details the composition of the live data to be evacuated. In the non-generational modes, all regions are considered young and nothing will be promoted (or old).

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve documentation for methods touched in this change

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27235/files
  - new: https://git.openjdk.org/jdk/pull/27235/files/aeb6b39e..e0854ecb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27235&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27235&range=04-05

  Stats: 11 lines in 2 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27235.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27235/head:pull/27235

PR: https://git.openjdk.org/jdk/pull/27235