EntityTest class should document how its bundles work

This patch adds a descriptive PHPDoc to explain that although the bundle key (type) is defined, no static bundles exist, and bundles can be dynamically created via EntityTestHelper::createBundle() in test cases.

This helps avoid confusion for contributors trying to reuse this test entity in their own test scenarios.

The text is rather too wordy, and also needs wrapping.

It reads like it's LLM-generated. Our policy requires documenting whenever AI is used to help craft code or documentation: https://www.drupal.org/docs/develop/issues/issue-procedures-and-etiquett... →

Created Patch

Latest patch has far too much detail. And reads even more like it's LLM-generated.

This issue is just about documenting the bundles.