nix-copy: document --all --from local binary cache example

src/nix/copy.md

@@ -11,6 +11,12 @@ R""(
   Note the `file://` - without this, the destination is a chroot
   store, not a binary cache.
 
+* Copy all store paths from a local binary cache in `/tmp/cache` to the local store:
+
+  ```console
+  # nix copy --all --from file:///tmp/cache
+  ```
+
 * Copy the entire current NixOS system closure to another machine via
   SSH: