Docs: Add missing descriptions for the $wpdb global in DocBlocks all the places.

See #32246.

Built from https://develop.svn.wordpress.org/trunk@35170


git-svn-id: http://core.svn.wordpress.org/trunk@35136 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -500,7 +500,7 @@ function wp_extract_urls( $content ) {
  *
  * @since 1.5.0
  *
- * @global wpdb $wpdb
+ * @global wpdb $wpdb WordPress database abstraction object.
  *
  * @param string $content Post Content.
  * @param int    $post_ID Post ID.
@@ -4317,7 +4317,7 @@ function _cleanup_header_comment( $str ) {
  *
  * @since 2.9.0
  *
- * @global wpdb $wpdb
+ * @global wpdb $wpdb WordPress database abstraction object.
  */
 function wp_scheduled_delete() {
 	global $wpdb;