Add overview description for ActiveRecord::Transactions, telling the reader to...

Add overview description for ActiveRecord::Transactions, telling the reader to look at ClassMethods instead.

activerecord/lib/active_record/transactions.rb

 require 'thread'
 
 module ActiveRecord
-  module Transactions # :nodoc:
+  # See ActiveRecord::Transactions::ClassMethods for documentation.
+  module Transactions
     class TransactionError < ActiveRecordError # :nodoc:
     end