Docs: toFile expects directory structure to exist

lib/output.js

@@ -28,6 +28,8 @@ const errMagickSave = new Error('GIF output requires libvips with support for Im
  * By default all metadata will be removed, which includes EXIF-based orientation.
  * See {@link withMetadata} for control over this.
  *
+ * The caller is responsible for ensuring directory structures and permissions exist.
+ *
  * A `Promise` is returned when `callback` is not provided.
  *
  * @example