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

Props jontyravi.
Fixes #51309.
Built from https://develop.svn.wordpress.org/trunk@48976


git-svn-id: http://core.svn.wordpress.org/trunk@48738 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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.

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-48975';
+$wp_version = '5.6-alpha-48976';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.