add description to ActiveModel::MissingAttributeError [ci skip]

activemodel/lib/active_model/attribute_methods.rb

@@ -3,6 +3,7 @@
 require 'active_support/deprecation'
 
 module ActiveModel
+  # Raised when an attribute is not defined.
   class MissingAttributeError < NoMethodError
   end
   # == Active Model Attribute Methods
@@ -279,7 +280,7 @@ def define_attribute_method(attr_name)
         attribute_method_matchers_cache.clear
       end
 
-      # Removes all the previously dynamically defined methods from the class
+      # Removes all the previously dynamically defined methods from the class.
       #
       #   class Person
       #     include ActiveModel::AttributeMethods