add example to ActiveModel::MissingAttributeError [ci skip]

activemodel/lib/active_model/attribute_methods.rb

@@ -3,6 +3,14 @@ require 'active_support/deprecation'
 
 module ActiveModel
   # Raised when an attribute is not defined.
+  #
+  #   class User < ActiveRecord::Base
+  #     has_many :pets
+  #   end
+  #
+  #   user = User.first
+  #   user.pets.select(:id).first.user_id
+  #   # => ActiveModel::MissingAttributeError: missing attribute: user_id
   class MissingAttributeError < NoMethodError
   end
   # == Active Model Attribute Methods