Fix up base/bind documentation - elaborate on reference binding.

base/callback.h

 // This file was GENERATED by command:
 //     pump.py callback.h.pump
 // DO NOT EDIT BY HAND!!!
 
 
 // Copyright (c) 2012 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_CALLBACK_H_
@@ skipping 251 matching lines @@
 //  - Performing compile-time asserts to avoid error-prone behavior
 //  - Returning an Callback<> with an arity matching the number of unbound
 //    parameters and that knows the correct refcounting semantics for the
 //    target object if we are binding a method.
 //
 // The Bind functions do the above using type-inference, and template
 // specializations.
 //
 // By default Bind() will store copies of all bound parameters, and attempt
 // to refcount a target object if the function being bound is a class method.
-// These copies are created even if the function takes parameters as const or
-// non-const references.
+// These copies are created even if the function takes parameters as const
+// references. (Binding to non-const references is forbidden, see bind.h)
 //
 // To change this behavior, we introduce a set of argument wrappers
 // (e.g., Unretained(), and ConstRef()).  These are simple container templates
 // that are passed by value, and wrap a pointer to argument.  See the
 // file-level comment in base/bind_helpers.h for more info.
 //
 // These types are passed to the Unwrap() functions, and the MaybeRefcount()
 // functions respectively to modify the behavior of Bind().  The Unwrap()
 // and MaybeRefcount() functions change behavior by doing partial
 // specialization based on whether or not a parameter is a wrapper type.
@@ skipping 467 matching lines @@
 };
 
 
 // Syntactic sugar to make Callbacks<void(void)> easier to declare since it
 // will be used in a lot of APIs with delayed execution.
 typedef Callback<void(void)> Closure;
 
 }  // namespace base
 
 #endif  // BASE_CALLBACK_H