RHL-005-fix(env): clarify root auth requirements in Docker environment configuration

.env.docker.example

@@ -5,7 +5,7 @@
 
 # MongoDB connection string inside the docker-compose network.
 # "db" should match the MongoDB service name in docker-compose.yml.
-# If root auth is enabled (MONGO_INITDB_ROOT_USERNAME/PASSWORD), include credentials.
+# If root auth is enabled, include credentials.
 MONGODB_URI=mongodb://root:replace-me@db:27017/rhl-lieferscheine?authSource=admin
 
 # Secret used to sign session JWT cookies.