Changed documentation to be more clear

Author: Floris-ll

wordpress-objects/Clarkson_Object.php

@@ -650,8 +650,9 @@ public function has_term( $term ) {
 	 * If you want something else, then just overwrite it in your own WordPress object.
 	 *
 	 * We can't just return $this->_post, because these values will only return raw unfiltered data.
+	 * @return array<string,mixed>
 	 */
-	public function jsonSerialize(): mixed {
+	public function jsonSerialize(): array {
 		$data['id']        = $this->get_id();
 		$data['link']      = $this->get_permalink();
 		$data['slug']      = $this->get_post_name();