Docs: Add reference to index numbers for cache clearing

doc/ci/caching/index.md

@@ -581,6 +581,9 @@ via the GitLab UI:
 
 1. On the next push, your CI/CD job uses a new cache.
 
+NOTE:
+Each time you clear the cache manually, the [internal cache name](#where-the-caches-are-stored) is updated. The name uses the format `cache-<index>`, and the index increments by one each time. The old cache is not deleted. You can manually delete these files from the runner storage.
+
 <!-- ## Troubleshooting
 
 Include any troubleshooting steps that you can foresee. If you know beforehand what issues