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

Props mt8.biz.
Fixes #38973.
Built from https://develop.svn.wordpress.org/trunk@39390


git-svn-id: http://core.svn.wordpress.org/trunk@39330 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/post.php

@@ -1160,6 +1160,8 @@ function wp_edit_attachments_query_vars( $q = false ) {
  * @since 4.7.0
  * @access private
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param array $clauses An array including WHERE, GROUP BY, JOIN, ORDER BY,
  *                       DISTINCT, fields (SELECT), and LIMITS clauses.
  * @return array The modified clauses.

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-39388';
+$wp_version = '4.8-alpha-39390';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.