Documentation: Clarify contradictory docs about authentication of replication traffic

[haslersn]

The workers documentation has seemingly contradictory statements about whether replication traffic is authenticated:

Optionally, a shared secret can be used to authenticate HTTP traffic between workers.

Under no circumstances should the replication listener be exposed to the public internet; it has no authentication

[richvdh]

yeah, the "no authentication" text predates #8853. Mind you, I'm not sure I'd really recommend exposing the HTTP port even if there is a shared secret.

Still, it's definitely incorrect as it stands.

[haslersn]

I'm not sure I'd really recommend exposing the HTTP port

Still, the authentication might influence a decision about whether it's acceptable to have unrelated hosts in the same VLAN or Kubernetes cluster.

[clokep]

#13656 is a simple clarification of this, not sure there is more to say?