Add details about anomaly rules

[alexane-bougeardbebin-sekoia]

See documentation: https://docs.sekoia.io/xdr/features/detect/anomaly/#aggregation-methods

See ticket: https://sekoia8055.zendesk.com/agent/tickets/9327?brand_id=9383708864797

See Copain + @r1chev answer: https://chat.sekoia.io/sekoia/pl/55ux61qijpfm5gmjzyemn6zpgh