Add tip for deepscrub usage

With this tip we want to make a suggestion about when using deepscrub, and
explaining that using it, it also flushes the cache.

Author: evallesp

docs/source/quickstart/05_clean_infra.md

@@ -25,3 +25,7 @@ In case you want to remove everything, with the base images.
     reproducer-clean.yml \
     --tags deepscrub
 ```
+
+~~~{tip}
+This includes flushing the Ansible cache. If you perform a quick cleanup and encounter new errors, you should run a deepscrub or re-run the playbook with --flush-cache.
+~~~