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

[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

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Commit messages:
 - Improve docs of CRaC VM options

Changes: https://git.openjdk.org/crac/pull/256/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364554
  Stats: 59 lines in 8 files changed: 5 ins; 0 del; 54 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