RFR: 8368681: Shenandoah: Add documentation comments for ShenandoahAllocationRate

[Kelvin Nilsen]

Add documentation to ShenandoahAllocationRate

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Commit messages:
 - Add documentation

Changes: https://git.openjdk.org/jdk/pull/27505/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27505&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368681
  Stats: 44 lines in 2 files changed: 44 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27505.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27505/head:pull/27505

PR: https://git.openjdk.org/jdk/pull/27505