Add notes about renderable option to guide

[swanson]

#39869 added the ability to use renderable as an option for render. This PR updates the guides to include this form of passing an object.

I needed to use this form of passing the object to work with Hotwire's broadcast_action_to method which is passing splatted args: https://github.com/hotwired/turbo-rails/blob/ea00f3732e21af9c2156cf74dabe95524b17c361/app/channels/turbo/streams/broadcasts.rb#L85-L87

[p8]

As render MyRenderable.new is basically syntactic sugar for render renderable: MyRenderable.new, maybe we should swap the examples and explain the :renderable option first?