Documentation: Clarification: Registering REST support for already registered CPT

[krogsgard]

Related to: http://v2.wp-api.org/extending/custom-content-types/

The rest_base and rest_controller_class only need to be set when adding to an already registered post type if they are not intended to use defaults. We should show a code sample just updating show_in_rest first, as to note how it's only a single argument to add support.

Just needs some restructuring on that page, as right now CPT support looks more difficult than it really is.