Add notes comment for isBinderAlive API, to warn developers who use this should know the limit of this API.

Change-Id: I365af33834cd2bf79487a185c7db743510abfe6c
diff --git a/core/java/android/os/BinderProxy.java b/core/java/android/os/BinderProxy.java
index 80546cd..3b5a99e 100644
--- a/core/java/android/os/BinderProxy.java
+++ b/core/java/android/os/BinderProxy.java
@@ -493,6 +493,11 @@
     public native boolean pingBinder();
 
     /**
+     * Check to see if the process that the binder is in is still alive.
+     *
+     * Note, this only reflects the last known death state, if the object
+     * is linked to death or has made a transactions since the death occurs.
+     *
      * @return false if the hosting process is gone
      */
     public native boolean isBinderAlive();