Apply LWG 2021. This is only a documentation change. Also bringing c++1y status page up to date.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191141 91177308-0d34-0410-b5e6-96231b3b80d8

include/future

@@ -309,11 +309,11 @@ public:
 };
 
 template <class F, class... Args>
-  future<typename result_of<F(Args...)>::type>
+  future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   async(F&& f, Args&&... args);
 
 template <class F, class... Args>
-  future<typename result_of<F(Args...)>::type>
+  future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   async(launch policy, F&& f, Args&&... args);
 
 template <class> class packaged_task; // undefined