RFR: 8246037: Shenandoah: update man pages to mention -XX:+UseShenandoahGC [v4]

[Aleksey Shipilev]

On Thu, 28 Aug 2025 00:15:58 GMT, Rui Li <duke at openjdk.org> wrote:

>> 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:
> 
>   gen shen and heuristics

Make sure that someone outside Shenandoah devs looks at this patch to sanity check how it reads.

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PR Comment: https://git.openjdk.org/jdk/pull/26907#issuecomment-3234456656