Update CLI documentation for `--cache-dir` (#8627)
Refs:
- #8626
## Summary
Current documentation incorrectly suggests that the macOS cache
directory location is `$HOME/Library/Caches/uv`, but that changed in:
- #5806
Updates docs to say this instead:
> <p>Defaults to <code>$HOME/.cache/uv</code> on macOS,
<code>$XDG_CACHE_HOME/uv</code> or <code>$HOME/.cache/uv</code> on
Linux, and <code>%LOCALAPPDATA%\uv\cache</code> on Windows. The <code>uv
cache dir</code> command will show the location of the cache
directory.</p>
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
