[DOC] Check warnings against multiple operands of the same type in the same namespace

[scholzj]

We seem to have quite a lot of users who run multiple Kafka clusters in the same namespace. We should double-check that the docs contain sufficient warnings against this in the important places to tell the users not to do this.

The reason is that various naming conflicts could occur and cause problems. This is most prominent for Kafka clusters, but ideally should be applied to other operands as well. (compared to that, running different operands in the same namespace might be less problematic, but maybe still something to ideally avoid?)