Clarify exactly how vague the spec for stop() really is.

pep-3156.txt

@@ -221,8 +221,13 @@ A conforming event loop object has the following methods:
 - ``stop()``.  Stops the event loop as soon as it is convenient.  It
   is fine to restart the loop with ``run()`` or ``run_until_complete()``
   subsequently; no scheduled callbacks will be lost if this happens.
-
-  Note: How soon the event loop stops is up to the implementation.
+  Note: ``stop()`` returns normally and the current callback is
+  allowed to continue.  How soon after this point the event loop stops
+  is up to the implementation, but the intention is to stop short of
+  polling for I/O, and not to run any callbacks scheduled in the
+  future; the major freedom an implementation has is how much of the
+  "ready queue" (callbacks already scheduled with ``call_soon()``) it
+  processes before stopping.
 
 - ``close()``.  Closes the event loop, releasing any resources it may
   hold, such as the file descriptor used by ``epoll()`` or