diff -r 7cca663a72eb Doc/library/asyncio-stream.rst
--- a/Doc/library/asyncio-stream.rst	Fri Mar 21 11:56:40 2014 +0100
+++ b/Doc/library/asyncio-stream.rst	Fri Mar 21 12:08:14 2014 +0100
@@ -87,6 +87,12 @@ StreamReader
 
 .. class:: StreamReader(limit=None, loop=None)
 
+   The optional *limit* parameter is the line length limit. It is a security
+   feature: a :exc:`ValueError` is raised if a line is too long. It also
+   doubles as half the buffer limit.
+
+   The default limit is 64 KB.
+
    .. method:: exception()
 
       Get the exception.