Index: src/wp-includes/wp-db.php
===================================================================
--- src/wp-includes/wp-db.php	(revision 28186)
+++ src/wp-includes/wp-db.php	(working copy)
@@ -1312,6 +1312,10 @@
 
 		$this->is_mysql = true;
 
+		/*
+		 * Deprecated in 3.9+ when using MySQLi. No equivalent
+		 * $new_link parameter exists for mysqli_* functions.
+		 */
 		$new_link = defined( 'MYSQL_NEW_LINK' ) ? MYSQL_NEW_LINK : true;
 		$client_flags = defined( 'MYSQL_CLIENT_FLAGS' ) ? MYSQL_CLIENT_FLAGS : 0;