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

Props mukesh27, utsav72640, immeet94, SergeyBiryukov.
See #45604.
Built from https://develop.svn.wordpress.org/trunk@45734


git-svn-id: http://core.svn.wordpress.org/trunk@45545 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/ms-functions.php

@@ -586,7 +586,7 @@ function wpmu_validate_user_signup( $user_name, $user_email ) {
  *
  * @since MU (3.0.0)
  *
- * @global wpdb   $wpdb
+ * @global wpdb   $wpdb   WordPress database abstraction object.
  * @global string $domain
  *
  * @param string         $blogname   The blog name provided by the user. Must be unique.