Document the odd SETTINGS behaviour on the window.

Right now we handle a window resize a bit strangely. For the moment this
interface is probably OK, but it won't stay OK forever.

Author: Lukasa

hyper/http20/window.py

@@ -67,7 +67,11 @@ def increase_window_size(self, frame_size):
         Note that this method is called before the window size is decremented
         as a result of the frame being handled.
 
-        :param frame_size: The size of the received frame.
+        :param frame_size: The size of the received frame. Note that this *may*
+          be zero. When this parameter is zero, it's possible that a
+          WINDOWUPDATE frame may want to be emitted anyway. A zero-length frame
+          size is usually associated with a change in the size of the receive
+          window due to a SETTINGS frame.
         :returns: The amount to increase the receive window by. Return zero if
           the window should not be increased.
         """