Clarify status of these notes.

src/ZEO/zrpc/NOTES

+[XXX These notes reflect a design that was never implemented]
+
 The Connection object should be extended to support more flexible
 handling for outstanding calls.  In particular, it should be possible
 to have multiple calls with return values outstanding.
@@ -76,4 +78,4 @@ Multithreaded server
 There are lots of issues to work out here.
 
 Delays may not be necessary if the connecftion handler runs in a
-different thread than the object the handles the calls.  
\ No newline at end of file
+different thread than the object the handles the calls.