RFR: 8246037: Shenandoah: update man pages to mention -XX:+UseShenandoahGC [v7]

[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`

Rui Li has updated the pull request incrementally with one additional commit since the last revision:

  update for threshold wording

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26907/files
  - new: https://git.openjdk.org/jdk/pull/26907/files/1c8d334a..1c25fbbb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26907&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26907&range=05-06

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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