Update deprecation message for matrix_playbook_ssl_retrieval_method

roles/custom/matrix_playbook_migration/tasks/validate_config.yml

@@ -308,7 +308,7 @@
         - {'old': 'matrix_prometheus_services_proxy_connect_prometheus_postgres_exporter_metrics_proxying_enabled', 'new': '<removed>'}
         - {'old': 'matrix_prometheus_services_proxy_connect_prometheus_postgres_exporter_matrix_nginx_proxy_not_enabled_proxy_pass_host', 'new': '<removed>'}
         - {'old': 'matrix_docker_network', 'new': '<removed in favor of various other variables - matrix_addons_container_network, matrix_monitoring_container_network, matrix_homeserver_container_network, etc.>'}
-        - {'old': 'matrix_playbook_ssl_retrieval_method', 'new': '<removed>'}
+        - {'old': 'matrix_playbook_ssl_retrieval_method', 'new': '<removed; see docs/configuring-playbook-ssl-certificates.md for alternatives>'}
         - {'old': 'matrix_ssl_lets_encrypt_support_email', 'new': 'devture_traefik_config_certificatesResolvers_acme_email'}
         - {'old': 'matrix_federation_traefik_entrypoint', 'new': 'matrix_federation_traefik_entrypoint_name'}