Trivial addition of 1-sentence description of missing param in docblock for Connection::update.

lib/Doctrine/DBAL/Connection.php

@@ -489,7 +489,7 @@ class Connection implements DriverConnection
      * Executes an SQL UPDATE statement on a table.
      *
      * @param string $tableName The name of the table to update.
-     * @param array $data
+     * @param array $data An associative array containing column-value pairs.
      * @param array $identifier The update criteria. An associative array containing column-value pairs.
      * @param array $types Types of the merged $data and $identifier arrays in that order.
      * @return integer The number of affected rows.