Docs: Consistently include an empty line between @since tag and @see, @link, or @global, per the documentation standards.

See #49572.

git-svn-id: https://develop.svn.wordpress.org/trunk@48067 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-functions.php

@@ -2200,6 +2200,7 @@ function add_existing_user_to_blog( $details = false ) {
  * is specifically hooked into the {@see 'wpmu_activate_user'} action.
  *
  * @since MU (3.0.0)
+ *
  * @see add_user_to_blog()
  *
  * @param int    $user_id  User ID.