Document that register_meta's show_in_rest for Custom Post Types requires custom-fields support

[kadamwhite]

As noted previously in #40408 and captured in a ​REST API handbook ticket, a custom post type must include custom-fields in its supports array or the meta key will not appear in REST API responses even if that meta is properly registered using register_meta.

This is true for both register_meta and the derivative register_post_meta method, as in the below rough example:

<?php
register_post_type( 'book', array(
    'public'       => true,
    'show_in_rest' => true,
    'supports'     => array( 'title', 'editor' ),
) );
register_post_meta( 'book', 'awards', array(
    'show_in_rest' => true,
    'single'       => true,
    'type'         => 'integer',
) );

The meta key will be missing from the REST API responses for the book post type in this example because custom-fields is not included in the supports array.
As meta over REST becomes more important to the block editor and complex plugins, I propose we expand the documentation of the show_in_rest argument for register_meta to highlight this caveat.

[kadamwhite]

Proposed documentation adjustment for show_in_rest field

[kadamwhite]

In 45786:

Docs: Clarify custom-fields support requirement for viewing custom post meta in REST API.

Fixes #47866.

[leogermani]

Great, I had to find out it myself couple of days ago and it wasn't straight forward.

As a suggestion, maybe it could indicate more directly what needs to be present. Something like: "... declare support to custom post types by having 'custom-fields' in its 'supports' array. (@see register_post_type)"