Add TBD about cancellation.

pep-3156.txt

@@ -750,6 +750,14 @@ coroutine when ``connection_made()`` is called.
 
 TBD: Be more specific.
 
+Cancellation
+------------
+
+TBD.  When a Task is canceled its coroutine may see an exception at
+any point where it is yielding to the scheduler (i.e., potentially at
+any ``yield from`` operation).  We need to spell out which exception
+is raised.
+
 
 Open Issues
 ===========
@@ -758,7 +766,8 @@ Open Issues
   (though not absolutely dictates) 'canceled' (one ell), but outside
   the US 'cancelled' (two ells) prevails.  PEP 3148, whose author
   currently lives in Australia, uses ``cancelled()`` as a method name
-  on its Future class.
+  on its Future class.  Also, even in the US, 'cancellation' seems
+  to be preferred over cancelation.
 
 - Do we need introspection APIs?  E.g. asking for the read callback
   given a file descriptor.  Or when the next scheduled call is.  Or