Docs: Improve documentation for WP_REST_Attachments_Controller.

This adds a missing DocBlock for `::register_routes()` and parameter descriptions for `::post_process_item()`.

Includes minor documenation fixes in other REST API classes for consistency.

Props dkarfa, mukesh27, SergeyBiryukov.
Fixes #48841.

git-svn-id: https://develop.svn.wordpress.org/trunk@47391 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php

@@ -40,6 +40,8 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 	 * Registers the routes for the objects of the controller.
 	 *
 	 * @since 4.7.0
+	 *
+	 * @see register_rest_route()
 	 */
 	public function register_routes() {