Clarify documentation about replication traffic. (#13656)

It can be authenticated with the worker_replication_secret setting,
but is always unencrypted.

changelog.d/13656.doc

+Clarify documentation that HTTP replication traffic can be protected with a shared secret.

docs/workers.md

@@ -120,7 +120,10 @@ redis:
 See the sample config for the full documentation of each option.
 
 Under **no circumstances** should the replication listener be exposed to the
-public internet; it has no authentication and is unencrypted.
+public internet; replication traffic is:
+
+* always unencrypted
+* unauthenticated, unless `worker_replication_secret` is configured
 
 
 ### Worker configuration