📝 docs(usage): document use_site_for_root parameter

docs/usage.rst

@@ -151,6 +151,24 @@ Defaults to ``False``.
    dirs = PlatformDirs("SuperApp", "Acme", ensure_exists=True)
    dirs.user_cache_dir  # directory is created if it does not exist
 
+``use_site_for_root``
+~~~~~~~~~~~~~~~~~~~~~
+
+Unix-only. When ``True``, redirects ``user_*_dir`` calls to their ``site_*_dir`` equivalents when
+running as root (uid 0). Defaults to ``False`` for backwards compatibility.
+
+When enabled, XDG user environment variables (e.g., ``XDG_DATA_HOME``) are bypassed for the
+redirected directories. This is useful for system services running as root that should use
+system-wide directories rather than root's home directory.
+
+.. code-block:: python
+
+   from platformdirs import PlatformDirs
+
+   dirs = PlatformDirs("SuperApp", use_site_for_root=True)
+   # When running as root, user_data_dir returns the site_data_dir path
+   dirs.user_data_dir  # Returns site directory instead of /root/.local/share/SuperApp
+
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