Update base::scoped_refptr<> docs

base/memory/ref_counted.h

Index: base/memory/ref_counted.h
diff --git a/base/memory/ref_counted.h b/base/memory/ref_counted.h
index 960c8a28453122a5e00a2cb72d5f420e7b642f15..784a1788a81a07808be26e82c830eeea59acc1b8 100644
--- a/base/memory/ref_counted.h
+++ b/base/memory/ref_counted.h
@@ -224,6 +224,9 @@ class RefCountedData
 //
 //   class MyFoo : public RefCounted<MyFoo> {
 //    ...
+//    private:
+//     friend class RefCounted<MyFoo>;  // Allow destruction by RefCounted<>.
+//     ~MyFoo();                        // Destructor must be private/protected.
 //   };
 //
 //   void some_function() {