doc: fix MacOS kernel disk caching wrt XDG_CACHE_HOME

[matthiasdiener]

See https://github.com/platformdirs/platformdirs/issues/4 and https://github.com/platformdirs/platformdirs/blob/7039c66b02f50fca54b965dcb2bd9de865d06cb8/src/platformdirs/macos.py#L56-L58 for context.

cc @MTCam , @inducer

Questions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?