document isBinderAlive

It's stale, came up in design doc. AIBinder_isAlive is already documented.

Change-Id: Ib8cf264bb692c39b917b1176697441a4153320bb
diff --git a/libs/binder/include/binder/IBinder.h b/libs/binder/include/binder/IBinder.h
index 17248ce..4eb1c08 100644
--- a/libs/binder/include/binder/IBinder.h
+++ b/libs/binder/include/binder/IBinder.h
@@ -102,6 +102,10 @@
      */
     virtual const String16& getInterfaceDescriptor() const = 0;
 
+    /**
+     * Last known alive status, from last call. May be arbitrarily stale.
+     * May be incorrect if a service returns an incorrect status code.
+     */
     virtual bool            isBinderAlive() const = 0;
     virtual status_t        pingBinder() = 0;
     virtual status_t        dump(int fd, const Vector<String16>& args) = 0;