[crac] RFR: 8364554: [CRaC] Improve CRaC VM options' docs [v5]

[Timofei Pushkin]

> 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

Timofei Pushkin has updated the pull request incrementally with two additional commits since the last revision:

 - Expire CRaC's special options since JDK 26
 - Replace Unix with POSIX

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Changes:
  - all: https://git.openjdk.org/crac/pull/256/files
  - new: https://git.openjdk.org/crac/pull/256/files/e661cdf3..a92d9616

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=256&range=04
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=256&range=03-04

  Stats: 12 lines in 2 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/crac/pull/256.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/256/head:pull/256

PR: https://git.openjdk.org/crac/pull/256