Adding documentation to clarify usage of attributes hash.

Serialization uses only the attributes hash's keys and calls
methods that are of the same name as the keys on the serialized
object.

activemodel/lib/active_model/serialization.rb

@@ -17,7 +17,7 @@ module ActiveModel
   #     attr_accessor :name
   #
   #     def attributes
-  #       {'name' => name}
+  #       {'name' => nil}
   #     end
   #
   #   end
@@ -29,8 +29,11 @@ module ActiveModel
   #   person.name = "Bob"
   #   person.serializable_hash   # => {"name"=>"Bob"}
   #
-  # You need to declare some sort of attributes hash which contains the attributes
-  # you want to serialize and their current value.
+  # You need to declare an attributes hash which contains the attributes
+  # you want to serialize. When called, serializable hash will use
+  # instance methods that match the name of the attributes hash's keys.
+  # In order to override this behavior, take a look at the private
+  # method read_attribute_for_serialization.
   #
   # Most of the time though, you will want to include the JSON or XML
   # serializations. Both of these modules automatically include the