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/general-template.php

@@ -1771,7 +1771,7 @@ function get_archives_link( $url, $text, $format = 'html', $before = '', $after
  *
  * @see get_archives_link()
  *
- * @global wpdb      $wpdb
+ * @global wpdb      $wpdb      WordPress database abstraction object.
  * @global WP_Locale $wp_locale
  *
  * @param string|array $args {
@@ -2041,7 +2041,7 @@ function calendar_week_mod( $num ) {
  *
  * @since 1.0.0
  *
- * @global wpdb      $wpdb
+ * @global wpdb      $wpdb      WordPress database abstraction object.
  * @global int       $m
  * @global int       $monthnum
  * @global int       $year