Add docs for ->insert_id. Props scribu. fixes #13439

git-svn-id: https://develop.svn.wordpress.org/trunk@14745 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/wp-db.php

@@ -107,6 +107,15 @@ class wpdb {
 	 */
 	var $rows_affected = 0;
 
+	/**
+	 * The ID generated for an AUTO_INCREMENT column by the previous query (usually INSERT).
+	 *
+	 * @since 0.71
+	 * @access public
+	 * @var int
+	 */
+	var $insert_id = 0;
+
 	/**
 	 * Saved result of the last query made
 	 *