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-deprecated.php

@@ -596,8 +596,8 @@ function insert_blog($domain, $path, $site_id) {
  * @since MU (3.0.0)
  * @deprecated 5.1.0
  *
- * @global wpdb     $wpdb
- * @global WP_Roles $wp_roles
+ * @global wpdb     $wpdb     WordPress database abstraction object.
+ * @global WP_Roles $wp_roles WordPress role management object.
  *
  * @param int    $blog_id    The value returned by wp_insert_site().
  * @param string $blog_title The title of the new site.