Improve metadata documentation

We should mention that you can now use `register_post_meta` and that custom post types need to have `custom-fields` support enabled.

Related Trac ticket: https://core.trac.wordpress.org/ticket/40408

https://github.com/WP-API/docs/blob/master/extending-the-rest-api/modifying-responses.md#working-with-registered-meta-in-the-rest-api