Docs: Add missing description for $wpdb, $wp_db_version, and $wp_current_db_version globals.

Props mukesh27, utsav72640, immeet94, SergeyBiryukov.
See #45604.

git-svn-id: https://develop.svn.wordpress.org/trunk@45734 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/load.php

@@ -406,7 +406,7 @@ function wp_set_lang_dir() {
  *
  * @since 2.5.0
  *
- * @global wpdb $wpdb The WordPress database class.
+ * @global wpdb $wpdb WordPress database abstraction object.
  */
 function require_wp_db() {
 	global $wpdb;
@@ -437,7 +437,7 @@ function require_wp_db() {
  * @since 3.0.0
  * @access private
  *
- * @global wpdb   $wpdb         The WordPress database class.
+ * @global wpdb   $wpdb         WordPress database abstraction object.
  * @global string $table_prefix The database table prefix.
  */
 function wp_set_wpdb_vars() {