Update base::scoped_refptr<> docs

base/memory/ref_counted.h

 // 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_MEMORY_REF_COUNTED_H_
 #define BASE_MEMORY_REF_COUNTED_H_
 
 #include <stddef.h>
 
 #include <cassert>
@@ skipping 206 matching lines @@
 }  // namespace base
 
 //
 // A smart pointer class for reference counted objects.  Use this class instead
 // of calling AddRef and Release manually on a reference counted object to
 // avoid common memory leaks caused by forgetting to Release an object
 // reference.  Sample usage:
 //
 //   class MyFoo : public RefCounted<MyFoo> {
 //    ...
+//    private:
+//     friend class RefCounted<MyFoo>;  // Allow destruction by RefCounted<>.
+//     ~MyFoo();                        // Destructor must be private/protected.
 //   };
 //
 //   void some_function() {
 //     scoped_refptr<MyFoo> foo = new MyFoo();
 //     foo->Method(param);
 //     // |foo| is released when this function returns
 //   }
 //
 //   void some_other_function() {
 //     scoped_refptr<MyFoo> foo = new MyFoo();
@@ skipping 215 matching lines @@
 bool operator!=(std::nullptr_t null, const scoped_refptr<T>& rhs) {
   return !operator==(null, rhs);
 }
 
 template <typename T>
 std::ostream& operator<<(std::ostream& out, const scoped_refptr<T>& p) {
   return out << p.get();
 }
 
 #endif  // BASE_MEMORY_REF_COUNTED_H_