Docs: Improve various globals documentation, as per documentation standards.

Props upadalavipul.
See #57069, #56792.


git-svn-id: https://develop.svn.wordpress.org/trunk@54953 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -3508,6 +3508,8 @@ function wp_destroy_all_sessions() {
  * @since 4.4.0
  * @since 4.9.0 The `$site_id` parameter was added to support multisite.
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param int|null $site_id Optional. The site ID to get users with no role for. Defaults to the current site.
  * @return string[] Array of user IDs as strings.
  */