pubsub: update documentation around Receive
@@ -407,7 +407,7 @@ var errReceiveInProgress = errors.New("pubsub: Receive already in progress for t
// The context passed to f will be canceled when ctx is Done or there is a
// fatal service error.
//
-// Receive will automatically extend the ack deadline of all fetched Messages for the
+// Receive will automatically extend the ack deadline of all fetched Messages up to the
// period specified by s.ReceiveSettings.MaxExtension.
//
// Each Subscription may have only one invocation of Receive active at a time.
