This adds `.attribute_names` and `#attribute_names` to
`ActiveModel::Attributes` along the same lines as the corresponding
methods in `ActiveRecord::AttributeMethods` (see
[`.attribute_names`][class_method] and
[`#attribute_names`][instance_method].

While I was here I also added documentation for '#attributes', which I
added in 043ce35b. The whole class is still `#:nodoc:` so I don't
think this will have any effect for now.

[class_method]: https://github.com/rails/rails/blob/cc834db1d0815744cfa173813c05d928e008e167/activerecord/lib/active_record/attribute_methods.rb#L154-L160
[instance_method]: https://github.com/rails/rails/blob/cc834db1d0815744cfa173813c05d928e008e167/activerecord/lib/active_record/attribute_methods.rb#L299-L301