fix(checkpoint): remove unimplemented options from help output

[nayuta723]

Description

Convert doc comments (///) to regular comments (//) for TODO options
and reference links in the Checkpoint struct in crates/liboci-cli/src/checkpoint.rs.

Clap uses doc comments to generate --help output, so unimplemented options
were incorrectly shown to users as if they were valid flags. This change hides
them while preserving the TODO notes for future implementation.

Also adds a missing doc comment for the container_id positional argument
so it appears correctly in help output.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test updates
• CI/CD related changes
• Other (please describe):

Testing

• Added new unit tests
• Added new integration tests
• Ran existing test suite
• Tested manually (please provide steps)

$ ./youki checkpoint --help
Checkpoint a running container

Usage: youki checkpoint [OPTIONS] <CONTAINER_ID>

Arguments:
  <CONTAINER_ID>  Identifier of the container

Options:
      --image-path <IMAGE_PATH>  Path for saving criu image files [default: checkpoint]
      --work-path <WORK_PATH>    Path for saving work files and logs
      --leave-running            Leave the process running after checkpointing
      --tcp-established          Allow open tcp connections
      --ext-unix-sk              Allow external unix sockets
      --shell-job                Allow shell jobs
      --file-locks               Allow file locks
  -h, --help                     Print help

Related Issues

Fixes #3453

Additional Context

[YJDoc2]

lgtm
One minor note, I feel container identifier to be a better than identifier of the container . Personal opinion ; fine with keeping existing or discussing wording more.