Document clean cache

docs/configuring-playbook-bot-matrix-registration-bot.md

@@ -59,3 +59,9 @@ In this room send `help` and the bot will reply with all options.
 You can also refer to the upstream [Usage documentation](https://github.com/moan0s/matrix-registration-bot#supported-commands).
 If you have any questions, or if you need help setting it up, read the [troublshooting guide](https://github.com/moan0s/matrix-registration-bot/blob/main/docs/troubleshooting.md)
 or join [#matrix-registration-bot:hyteck.de](https://matrix.to/#/#matrix-registration-bot:hyteck.de).
+
+To clean the cache (session&encryption data) after you changed the bot's username, changed the login methon form access_token to password etc.. you can use
+
+```bash
+just run-tags bot-matrix-registration-bot-clean-cache
+```