add some doc about the run-docker-prune tag

docs/maintenance-and-troubleshooting.md

@@ -33,6 +33,12 @@ matrix_synapse_root_log_level: "INFO"
 
 Re-run the playbook after making these configuration changes.
 
+## Remove unused Docker data
+
+You can free some disk space from Docker, see [docker system prune](https://docs.docker.com/engine/reference/commandline/system_prune/) for more information.
+```bash
+ansible-playbook -i inventory/hosts setup.yml --tags=run-docker-prune
+```
 
 ## Postgres