update AMo::ForbiddenAttributesError documentation [ci skip]

activemodel/lib/active_model/forbidden_attributes_protection.rb

@@ -1,4 +1,16 @@
 module ActiveModel
+  # Raised when forbidden attributes are used for mass assignment.
+  #
+  #   class Person < ActiveRecord::Base
+  #   end
+  #
+  #   params = ActionController::Parameters.new(name: 'Bob')
+  #   Person.new(params)
+  #   # => ActiveModel::ForbiddenAttributesError
+  #
+  #   params.permit!
+  #   Person.new(params)
+  #   # => #<Person id: nil, name: "Bob">
   class ForbiddenAttributesError < StandardError
   end