Docs: Improve various globals documentation, as per documentation standards.

Props upadalavipul.
See #57069, #56792.


git-svn-id: https://develop.svn.wordpress.org/trunk@54953 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -7953,6 +7953,8 @@ function wp_add_trashed_suffix_to_post_name_for_trashed_posts( $post_name, $post
  * @since 4.5.0
  * @access private
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param WP_Post $post The post.
  * @return string New slug for the post.
  */