Document why we have to check with defined() first.

lib/private/db/connectionfactory.php

@@ -61,6 +61,8 @@ class ConnectionFactory {
 			throw new \InvalidArgumentException("Unsupported type: $type");
 		}
 		$result = $this->defaultConnectionParams[$normalizedType];
+		// \PDO::MYSQL_ATTR_FOUND_ROWS may not be defined, e.g. when the MySQL
+		// driver is missing. In this case, we won't be able to connect anyway.
 		if ($normalizedType === 'mysql' && defined('\PDO::MYSQL_ATTR_FOUND_ROWS')) {
 			$result['driverOptions'] = array(
 				\PDO::MYSQL_ATTR_FOUND_ROWS => true,