Docs: Clarify that wp_list_pages() and get_pages() only work with hierarchical post types.

Props doublehhh.
Fixes #39369.

git-svn-id: https://develop.svn.wordpress.org/trunk@40027 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -4471,7 +4471,7 @@ function get_page_uri( $page = 0 ) {
 }
 
 /**
- * Retrieve a list of pages.
+ * Retrieve a list of pages (or hierarchical post type items).
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *