1. KAFKA-19563 is created for this issue, and upstream doc will be updated first (PR).

slack discussion: https://redhat-internal.slack.com/archives/C067V3MQP2S/p1753818722886979

2. When searching for the add/remove controller doc, I found the TOC is confusing:

Before I expand the 14 section, I thought it is for brokers scaling only, not for controller. But it includes the controller scaling. We need to make it clear.