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/ms-functions.php

@@ -1279,6 +1279,8 @@ function wpmu_activate_signup( $key ) {
  *
  * @since 5.5.0
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param int      $id       ID of the user to delete.
  * @param int|null $reassign ID of the user to reassign posts and links to.
  * @param WP_User  $user     User object.