Add documentation about how to use max-distance with Slurm v6

[nick-stroud]

Testing:

• Tested using c2d-standard-2 with placement on and off
• confirmed when plugin on and placement on that max distance was set

Submission Checklist

Please take the following actions before submitting this pull request.

• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cloud HPC Toolkit Contribution guidelines #