libbinder: doc RpcServer::join/shutdown behavior

There have been a few changes to these recently, and the documentation
has gotten out of date.

Bug: 185167543
Test: N/A
Change-Id: Id4971e54fe0bfd175c042817f61e524765c03527
diff --git a/libs/binder/include/binder/RpcServer.h b/libs/binder/include/binder/RpcServer.h
index 178459d..a08c401 100644
--- a/libs/binder/include/binder/RpcServer.h
+++ b/libs/binder/include/binder/RpcServer.h
@@ -127,6 +127,10 @@
      * If a client needs to actively terminate join, call shutdown() in a separate thread.
      *
      * At any given point, there can only be one thread calling join().
+     *
+     * Warning: if shutdown is called, this will return while the shutdown is
+     * still occurring. To ensure that the service is fully shutdown, you might
+     * want to call shutdown after 'join' returns.
      */
     void join();
 
@@ -135,7 +139,7 @@
      * (e.g. no join() is running). Will wait for the server to be fully
      * shutdown.
      *
-     * TODO(b/185167543): wait for sessions to shutdown as well
+     * Warning: this will hang if it is called from its own thread.
      */
     [[nodiscard]] bool shutdown();