add :nodoc: directives to AMo::DeprecatedMassAssignmentSecurity [ci skip]

activemodel/lib/active_model/deprecated_mass_assignment_security.rb

 module ActiveModel
-  module DeprecatedMassAssignmentSecurity
+  module DeprecatedMassAssignmentSecurity # :nodoc:
     extend ActiveSupport::Concern
 
      module ClassMethods
-       def attr_protected(*args)
+       def attr_protected(*args) # :nodoc:
          raise "`attr_protected` is extracted out of Rails into a gem. " \
            "Please use new recommended protection model for params " \
            "or add `protected_attributes` to your Gemfile to use old one."
        end
 
-       def attr_accessible(*args)
+       def attr_accessible(*args) # :nodoc:
          raise "`attr_accessible` is extracted out of Rails into a gem. " \
            "Please use new recommended protection model for params " \
            "or add `protected_attributes` to your Gemfile to use old one."