Add some comments to base.

base/hash_tables.h

 // Copyright (c) 2006-2008 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.
 //
 
 //
 // Deal with the differences between Microsoft and GNU implemenations
 // of hash_map. Allows all platforms to use |base::hash_map| and
 // |base::hash_set|.
 //  eg:
 //   base::hash_map<int> my_map;
 //   base::hash_set<int> my_set;
 //
+// NOTE: It is an explicit non-goal of this class to provide a generic hash
+// function for pointers.  If you to hash a pointers to a particular class,

[darin (slow to review), 2009/09/03 05:23:11]

nit: "If you _want_ to hash pointers..."

+// please define the template specialization elsewhere (for example, in its
+// header file) and keep it specific to just pointers to that class.  This is
+// because identity hashes are not desirable for all types that might show up
+// in containers as pointers.
 
 #ifndef BASE_HASH_TABLES_H_
 #define BASE_HASH_TABLES_H_
 
 #include "build/build_config.h"
 
 #include "base/string16.h"
 
 #if defined(COMPILER_MSVC)
 #include <hash_map>
@@ skipping 71 matching lines @@
 DEFINE_STRING_HASH(string16);
 #endif  // WCHAR_T_IS_UTF32
 
 #undef DEFINE_STRING_HASH
 
 }  // namespace __gnu_cxx
 
 #endif  // COMPILER
 
 #endif  // BASE_HASH_TABLES_H_