Document the headers parameter for set_tunnel.
diff --git a/Doc/library/httplib.rst b/Doc/library/httplib.rst
index bf7e8ef..d881968 100644
--- a/Doc/library/httplib.rst
+++ b/Doc/library/httplib.rst
@@ -428,11 +428,14 @@
debug level is ``0``, meaning no debugging output is printed.
-.. method:: HTTPConnection.set_tunnel(host,port=None)
+.. method:: HTTPConnection.set_tunnel(host,port=None, headers=None)
Set the host and the port for HTTP Connect Tunnelling. Normally used when
it is required to do HTTPS Conection through a proxy server.
+ The headers argument should be a mapping of extra HTTP headers to to sent
+ with the CONNECT request.
+
.. versionadded:: 2.7