1.x: fix SafeSubscriber documentation regarding unsubscribe

The documentation was wrong in two points: unsubscription doesn't call
onCompleted and unsubscription doesn't directly prevent delivery of
onXXX events since the implementation doesn't even check isUnsubscribed:
(it is the responsibility of the upstream to do that).

src/main/java/rx/observers/SafeSubscriber.java

@@ -50,7 +50,8 @@
  * <ul>
  * <li>Allows only single execution of either {@code onError} or {@code onCompleted}.</li>
  * <li>Ensures that once an {@code onCompleted} or {@code onError} is performed, no further calls can be executed</li>
- * <li>If {@code unsubscribe} is called, calls {@code onCompleted} and forbids any further {@code onNext} calls.</li>
+ * <li>If {@code unsubscribe} is called, the upstream {@code Observable} is notified and the event delivery will be stopped in a
+ * best effort manner (i.e., further onXXX calls may still slip through).</li>
  * <li>When {@code onError} or {@code onCompleted} occur, unsubscribes from the {@code Observable} (if executing asynchronously).</li>
  * </ul>
  * {@code SafeSubscriber} will not synchronize {@code onNext} execution. Use {@link SerializedSubscriber} to do