improve explanation of 'max_per_second' option.

Author: gassan

mailer.rst

@@ -273,7 +273,7 @@ Other Options
         $dsn = 'smtps://smtp.example.com?ping_threshold=200'
 
 ``max_per_second``
-    The number of messages to send per second (0 to disable)::
+    The number of messages to send per second (0 to disable this limitation)::
 
         $dsn = 'smtps://smtp.example.com?max_per_second=2'