add links to driver exception code reference

lib/Doctrine/DBAL/Driver/Mysqli/Driver.php

@@ -76,6 +76,9 @@ class Driver implements DriverInterface, ExceptionConverterDriver
 
     /**
      * {@inheritdoc}
+     *
+     * @link http://dev.mysql.com/doc/refman/5.7/en/error-messages-client.html
+     * @link http://dev.mysql.com/doc/refman/5.7/en/error-messages-server.html
      */
     public function convertExceptionCode(\Exception $exception)
     {

lib/Doctrine/DBAL/Driver/PDOMySql/Driver.php

@@ -118,6 +118,9 @@ class Driver implements \Doctrine\DBAL\Driver, ExceptionConverterDriver
 
     /**
      * {@inheritdoc}
+     *
+     * @link http://dev.mysql.com/doc/refman/5.7/en/error-messages-client.html
+     * @link http://dev.mysql.com/doc/refman/5.7/en/error-messages-server.html
      */
     public function convertExceptionCode(\Exception $exception)
     {

lib/Doctrine/DBAL/Driver/PDOPgSql/Driver.php

@@ -109,6 +109,8 @@ class Driver implements \Doctrine\DBAL\Driver, ExceptionConverterDriver
 
     /**
      * {@inheritdoc}
+     *
+     * @link http://www.postgresql.org/docs/9.3/static/errcodes-appendix.html
      */
     public function convertExceptionCode(\Exception $exception)
     {

lib/Doctrine/DBAL/Driver/PDOSqlite/Driver.php

@@ -123,6 +123,8 @@ class Driver implements \Doctrine\DBAL\Driver, ExceptionConverterDriver
 
     /**
      * {@inheritdoc}
+     *
+     * @link http://www.sqlite.org/c3ref/c_abort.html
      */
     public function convertExceptionCode(\Exception $exception)
     {

lib/Doctrine/DBAL/Driver/SQLAnywhere/Driver.php

@@ -74,6 +74,8 @@ class Driver implements \Doctrine\DBAL\Driver, ExceptionConverterDriver
 
     /**
      * {@inheritdoc}
+     *
+     * @link http://dcx.sybase.com/index.html#sa160/en/saerrors/sqlerror.html
      */
     public function convertExceptionCode(\Exception $exception)
     {