Move documentation of base::Callback from base/callback.h to docs/callback.md

base/bind.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_BIND_H_
 #define BASE_BIND_H_
 
 #include "base/bind_internal.h"
 
 // -----------------------------------------------------------------------------
 // Usage documentation
 // -----------------------------------------------------------------------------
 //
-// See base/callback.h for documentation.
+// See //docs/callback.md for documentation.
 //
 //
 // -----------------------------------------------------------------------------
 // Implementation notes
 // -----------------------------------------------------------------------------
 //
 // If you're reading the implementation, before proceeding further, you should
 // read the top comment of base/bind_internal.h for a definition of common
 // terms and concepts.
 
 namespace base {
 
 template <typename Functor, typename... Args>
 inline base::Callback<MakeUnboundRunType<Functor, Args...>> Bind(
     Functor&& functor,
     Args&&... args) {
   using BindState = internal::MakeBindStateType<Functor, Args...>;
   using UnboundRunType = MakeUnboundRunType<Functor, Args...>;
   using Invoker = internal::Invoker<BindState, UnboundRunType>;
 
   using CallbackType = Callback<UnboundRunType>;
   return CallbackType(new BindState(std::forward<Functor>(functor),
                                     std::forward<Args>(args)...),
                       &Invoker::Run);
 }
 
 }  // namespace base
 
 #endif  // BASE_BIND_H_