Docs: Document the usage of $wpdb global in wp_delete_attachment_files().

Props jontyravi.
Fixes #51309.

git-svn-id: https://develop.svn.wordpress.org/trunk@48976 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -5855,6 +5855,8 @@ function wp_delete_attachment( $post_id, $force_delete = false ) {
  *
  * @since 4.9.7
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param int    $post_id      Attachment ID.
  * @param array  $meta         The attachment's meta data.
  * @param array  $backup_sizes The meta data for the attachment's backup images.