Add missing @return tag to gutenberg_meta_box_save_redirect() fun…

…ction (#11001)

The `@return` tag is missing in the docblock of the `gutenberg_meta_box_save_redirect()` function.

The function returns the modified location.

lib/meta-box-partial-page.php

@@ -57,6 +57,7 @@ function gutenberg_meta_box_save() {
  *
  * @param string $location The location of the meta box, 'side', 'normal'.
  * @param int    $post_id  Post ID.
+ * @return string Modified location of the meta box.
  *
  * @hooked redirect_post_location priority 10
  */