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

Merge of [39390] to the 4.7 branch.

Props mt8.biz.
See #38973.

git-svn-id: https://develop.svn.wordpress.org/branches/4.7@39391 602fd350-edb4-49c9-b593-d223f7449a82

src/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.