RFR: 8246037: Shenandoah: update man pages to mention -XX:+UseShenandoahGC [v3]

[William Kemper]

On Tue, 26 Aug 2025 22:53:07 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:
> 
>   Update for comments

Changes requested by wkemper (Reviewer).

src/java.base/share/man/java.md line 2907:

> 2905:         [JEP 521](https://openjdk.org/jeps/521) for its advantages and risks.
> 2906: 
> 2907: `-XX:ShenandoahGCHeuristics=`*heuristics*

Should probably mention that the generational mode ignores (and now issues a warning) when anything other than `adaptive` is selected.

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PR Review: https://git.openjdk.org/jdk/pull/26907#pullrequestreview-3162122639
PR Review Comment: https://git.openjdk.org/jdk/pull/26907#discussion_r2305543511