[crac] RFR: 8364554: [CRaC] Improve CRaC VM options' docs

[Timofei Pushkin]

On Fri, 1 Aug 2025 19:22:03 GMT, Timofei Pushkin <tpushkin at openjdk.org> wrote:

> Besides some wording and formatting of VM options:
> - Renames `CRaCAllowToSkipCheckpoint` to `CRaCSkipCheckpoint` since it makes the VM always skip checkpoints and not only in some cases
> - Replaces `CRaCMaxHeapSizeBeforeCheckpoint == 0` with `FLAG_IS_DEFAULT(CRaCMaxHeapSizeBeforeCheckpoint)` because the docs 0 do not specify 0 as a special value

src/java.base/share/man/java.md line 1090:

> 1088: `-XX:CRaCMinPid=`*value*
> 1089: :   A desired minimal PID value for checkpointed process. Used by the launcher,
> 1090:     only on POSIX-like platforms.

BTW, I thought about removing `CRaCMinPid` from `globals.hpp` since it is not used by the VM at all but decided against it (too big of a change for a pretty general PR like this) and just documented that it is to be used by the launcher.

Since it is (or should be) possible to use JVM without the default launcher we could add a check in VM that the option is honored and at least warn if it is not.

We've had similar problems with Tomcat launching JVM directly and thus not honoring options implemented purely in the launcher (see ZULU-79238).

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PR Review Comment: https://git.openjdk.org/crac/pull/256#discussion_r2249244467