Docs: Add a @global entry for $wpdb in is_user_member_of_blog().

Props ajayghaghretiya1.
Fixes #41308.

git-svn-id: https://develop.svn.wordpress.org/trunk@41039 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -694,6 +694,8 @@ function get_blogs_of_user( $user_id, $all = false ) {
  *
  * @since MU 1.1
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param int $user_id Optional. The unique ID of the user. Defaults to the current user.
  * @param int $blog_id Optional. ID of the blog to check. Defaults to the current site.
  * @return bool