Improve comments in \Drupal\router_test\TestControllers

Problem/Motivation

The sniff Drupal.Commenting.FunctionComment.Missing is ignore for core/modules/system/tests/modules/router_test_directory/src/TestControllers.php

Steps to reproduce

Proposed resolution

Add function comments as needed to the file

Remaining tasks

Create MR
Review

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet