Update: Followups to the template and template_lock rest api addition.

[jorgefilipecosta]

This PR does some follow-ups to #62488.

Namely improvement of the description in the schema as suggested by @mcsf.
Adds the missing backport changelog referred to by @ellatrix.

Testing

I added the following test code to a PHP file:

function myplugin_register_template() {
    $post_type_object = get_post_type_object( 'page' );
    $post_type_object->template = array(
        array( 'core/image' ),
    );
	$post_type_object->template_lock = 'all';
}
add_action( 'init', 'myplugin_register_template' );

I opened the site editor and pasted:
wp.data.select('core').getPostType( 'page' );

I verified that the template and tempalte_lock properties are still included in the response as expected.

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Co-authored-by: jorgefilipecosta <jorgefilipecosta@git.wordpress.org>
Co-authored-by: ellatrix <ellatrix@git.wordpress.org>

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[ellatrix]

I just cherry-picked this PR to the try/6.6-rc-1 branch to get it included in the next release: 93919ed