Docs: Document the VALID_ORIGINS constant in WP_Theme_JSON.

Follow-up to [51149].

See #52628.

git-svn-id: https://develop.svn.wordpress.org/trunk@51152 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -40,6 +40,12 @@ class WP_Theme_JSON {
 	 */
 	const ROOT_BLOCK_SELECTOR = 'body';
 
+	/**
+	 * The sources of data this object can represent.
+	 *
+	 * @since 5.8.0
+	 * @var array
+	 */
 	const VALID_ORIGINS = array(
 		'core',
 		'theme',