Fix the method signature for transaction helper in CollectionAssociation

The arguments for `transaction` should be `**options`, actually `*args`
is unused though.

And also, the comment is not for the method anymore, the method is not
called from public.

Author: kamipo

activerecord/lib/active_record/associations/collection_association.rb

@@ -121,21 +121,6 @@ def concat(*records)
         end
       end
 
-      # Starts a transaction in the association class's database connection.
-      #
-      #   class Author < ActiveRecord::Base
-      #     has_many :books
-      #   end
-      #
-      #   Author.first.books.transaction do
-      #     # same effect as calling Book.transaction
-      #   end
-      def transaction(*args)
-        reflection.klass.transaction(*args) do
-          yield
-        end
-      end
-
       # Removes all records from the association without calling callbacks
       # on the associated records. It honors the +:dependent+ option. However
       # if the +:dependent+ value is +:destroy+ then in that case the +:delete_all+
@@ -315,6 +300,10 @@ def find_from_target?
       end
 
       private
+        def transaction(&block)
+          reflection.klass.transaction(&block)
+        end
+
         # We have some records loaded from the database (persisted) and some that are
         # in-memory (memory). The same record may be represented in the persisted array
         # and in the memory array.