expand policy spec s3 docstring

Author: relh

packages/mettagrid/python/src/mettagrid/policy/prepare_policy_spec.py

@@ -186,9 +186,19 @@ def load_policy_spec_from_s3(
     *,
     device: str | None = None,
 ) -> PolicySpec:
-    """Download a submission archive or checkpoint dir from S3 into cache and return a PolicySpec.
+    """Download a submission archive or checkpoint dir from S3 and return a PolicySpec ready for loading.
 
-    Caches by URI hash; applies device override when provided.
+    Downloads into a deterministic cache location based on the URI hash, allowing
+    reuse across calls with the same URI.
+
+    Args:
+        s3_path: S3 path to a submission archive (.zip) or checkpoint directory
+        cache_dir: Base directory for caching. Defaults to /tmp/mettagrid-policy-cache
+        remove_downloaded_copy_on_exit: If True, register an atexit handler to clean up the cache directory
+        device: Override the device in the loaded spec (e.g., "cpu" or "cuda:0")
+
+    Returns:
+        PolicySpec with paths resolved to the local extraction directory
     """
     if cache_dir is None:
         cache_dir = DEFAULT_POLICY_CACHE_DIR