Document message_body arg in HTTPConnection.endheaders

Doc/library/http.client.rst

@@ -472,9 +472,13 @@ also send your request step by step, by using the four functions below.
    an argument.
 
 
-.. method:: HTTPConnection.endheaders()
+.. method:: HTTPConnection.endheaders(message_body=None)
 
-   Send a blank line to the server, signalling the end of the headers.
+   Send a blank line to the server, signalling the end of the headers. The
+   optional message_body argument can be used to pass message body
+   associated with the request.  The message body will be sent in
+   the same packet as the message headers if possible.  The
+   message_body should be a string.
 
 
 .. method:: HTTPConnection.send(data)