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>

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-cache/src/cli.rs

@@ -23,8 +23,10 @@ pub struct CacheArgs {
 
     /// Path to the cache directory.
     ///
-    /// Defaults to `$HOME/Library/Caches/uv` on macOS, `$XDG_CACHE_HOME/uv` or `$HOME/.cache/uv` on
-    /// Linux, and `%LOCALAPPDATA%\uv\cache` on Windows.
+    /// Defaults to `$XDG_CACHE_HOME/uv` or `$HOME/.cache/uv` on macOS and Linux, and
+    /// `%LOCALAPPDATA%\uv\cache` on Windows.
+    ///
+    /// To view the location of the cache directory, run `uv cache dir`.
     #[arg(global = true, long, env = EnvVars::UV_CACHE_DIR)]
     pub cache_dir: Option<PathBuf>,
 }