added docs to alias_attribute method

activemodel/lib/active_model/attribute_methods.rb

@@ -180,6 +180,37 @@ module ActiveModel
         undefine_attribute_methods
       end
 
+
+      # Allows you to make aliases for attributes.
+      #
+      # For example:
+      #
+      #   class Person
+      #
+      #     include ActiveModel::AttributeMethods
+      #     attr_accessor :name
+      #     attribute_method_prefix 'clear_'
+      #
+      #     define_attribute_methods [:name]
+      #
+      #     private
+      #
+      #     def clear_attribute(attr)
+      #       send("#{attr}=", nil)
+      #     end
+      #   end
+      #
+      #   class Person
+      #     attr_accessor :nickname
+      #
+      #     alias_attribute :nickname, :name
+      #   end
+      #
+      #   person = Person.new
+      #   person.nickname = "Bob"
+      #   person.nickname # => "Bob"
+      #   person.clear_nickname
+      #   person.nickname # => nil
       def alias_attribute(new_name, old_name)
         attribute_method_matchers.each do |matcher|
           matcher_new = matcher.method_name(new_name).to_s