[jdk25u-dev] RFR: 8246037: Shenandoah: update man pages to mention -XX:+UseShenandoahGC

[William Kemper]

8246037: Shenandoah: update man pages to mention -XX:+UseShenandoahGC

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Commit messages:
 - Backport 5052a7eee57e9d145950a0ab1ca71edc02bfe0be

Changes: https://git.openjdk.org/jdk25u-dev/pull/331/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u-dev&pr=331&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8246037
  Stats: 44 lines in 2 files changed: 42 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk25u-dev/pull/331.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u-dev.git pull/331/head:pull/331

PR: https://git.openjdk.org/jdk25u-dev/pull/331