Docs: Correct documentation for wp_get_post_parent_id().

The `$post` parameter is required and does not have a default value.

Follow-up to [42397].

Props denisco.
See #53399.

git-svn-id: https://develop.svn.wordpress.org/trunk@51425 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -7437,7 +7437,7 @@ function _publish_post_hook( $post_id ) {
  *
  * @since 3.1.0
  *
- * @param int|WP_Post $post Post ID or post object. Defaults to global $post.
+ * @param int|WP_Post $post Post ID or post object.
  * @return int|false Post parent ID (which can be 0 if there is no parent),
  *                   or false if the post does not exist.
  */