Clarify callback.h const reference binding documentation.

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 196 matching lines @@
 //
 //   void TakesOneRef(scoped_refptr<Foo> arg) {}
 //   scoped_refptr<Foo> f(new Foo)
 //   base::Closure cb = base::Bind(&TakesOneRef, f);
 //
 //   This should "just work." The closure will take a reference as long as it
 //   is alive, and another reference will be taken for the called function.
 //
 // PASSING PARAMETERS BY REFERENCE
 //
-//   void foo(int arg) { cout << arg << endl }
+//   Const references are *copied* unless ConstRef is used. Example:
+//
+//   void foo(const int& arg) { printf("%d %p\n", arg, &arg); }
 //   int n = 1;
+//   base::Closure has_copy = base::Bind(&foo, n);
 //   base::Closure has_ref = base::Bind(&foo, base::ConstRef(n));
 //   n = 2;
-//   has_ref.Run();  // Prints "2"
+//   foo(n);                        // Prints "2 0xaaaaaaaaaaaa"
+//   has_copy.Run();                // Prints "1 0xbbbbbbbbbbbb"
+//   has_ref.Run();                 // Prints "2 0xaaaaaaaaaaaa"
 //
 //   Normally parameters are copied in the closure. DANGER: ConstRef stores a
 //   const reference instead, referencing the original parameter. This means
 //   that you must ensure the object outlives the callback!
 //
 //
 // -----------------------------------------------------------------------------
 // Implementation notes
 // -----------------------------------------------------------------------------
 //
@@ skipping 524 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