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

[Rui Li]

Add documentation of Shenandoah to java man page

Aside from `-XX:+UseShenandoahGC`, I picked flags from [shenandoah_globals.hpp](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp) that are at product level but not experimental / diagnostic to avoid overwhelming info. Two additional flags match: `ShenandoahGCMode` and `ShenandoahGCHeuristics`

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

Changes: https://git.openjdk.org/jdk/pull/26907/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26907&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8246037
  Stats: 41 lines in 1 file changed: 41 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26907.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26907/head:pull/26907

PR: https://git.openjdk.org/jdk/pull/26907