Add quick reference for bind/callback usage.

base/bind.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
 $$
 $$ MAX_ARITY controls the number of arguments that Bind() supports.
 $$ The amount of code, and more importantly, the number of template types
@@ skipping 18 matching lines @@
 // 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_
 #pragma once
 
 #include "base/bind_internal.h"
 #include "base/callback_internal.h"
 
-// See base/callback.h for how to use these functions. If 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.
+// -----------------------------------------------------------------------------
+// User documentation
+// -----------------------------------------------------------------------------
 //
-// IMPLEMENTATION NOTE
+// See base/callback.h 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.
+//
+// RETURN TYPES
+//
 // Though Bind()'s result is meant to be stored in a Callback<> type, it
 // cannot actually return the exact type without requiring a large amount
 // of extra template specializations. The problem is that in order to
 // discern the correct specialization of Callback<>, Bind would need to
 // unwrap the function signature to determine the signature's arity, and
 // whether or not it is a method.
 //
 // Each unique combination of (arity, function_type, num_prebound) where
 // function_type is one of {function, method, const_method} would require
 // one specialization.  We eventually have to do a similar number of
@@ skipping 79 matching lines @@
   return Callback<typename BindState::UnboundRunType>(
       new BindState(internal::MakeRunnable(functor)[[]]
 $if ARITY > 0 [[, ]] $for ARG , [[p$(ARG)]]));
 }
 
 ]]  $$ for ARITY
 
 }  // namespace base
 
 #endif  // BASE_BIND_H_