Document events being emitted for content flow actions

[kwin]

I would expect that long-running operations emit events in order to be notified upon completion or failure (otherwise one would need to rely on polling with https://developer.adobe.com/experience-cloud/cloud-manager/reference/api/#tag/ContentSets/operation/listContentFlows). Currently https://developer.adobe.com/experience-cloud/cloud-manager/reference/events/ only list the events for pipeline step operations.