MongoDB: Missing documentation on authorization and internal authentication in ReplicaSet

Question

I stumbled on a problem transforming a standalone Mongo instance to a replicaset. The standalone already had authorization enabled.When I transformed it to a replicaset and started adding the members, it failed.

Googling around, I found that I needed to setup internal authentication with keyfiles for this. But I cant find in the documentation where it says that this is mandatory.I found that enabling internal authentication also enables authorisation, but not the other way around.

Have I found a missing piece in the documentation or is there a piece in the documentation that I have not found yet?