Merge pull request #342 from mghicho/buffer_size_fixed

Fixed the docstring for the set_buffer_size function. 
diff --git a/serial/serialwin32.py b/serial/serialwin32.py
index 74bfd05..bd1944c 100644
--- a/serial/serialwin32.py
+++ b/serial/serialwin32.py
@@ -418,7 +418,7 @@
     def set_buffer_size(self, rx_size=4096, tx_size=None):
         """\
         Recommend a buffer size to the driver (device driver can ignore this
-        value). Must be called before the port is opened.
+        value). Must be called after the port is opened.
         """
         if tx_size is None:
             tx_size = rx_size