add some comment

shared/db/postgres.nix

@@ -30,6 +30,7 @@ in
     };
 
     systemd.services.postgresql.environment = lib.mkIf (builtins.elem myName mapping.db.slaves) {
+        # Currently unused cause pwd is mannually set during basebackup
         PGPASSFILE = "${config.age.secrets.repli.path}";
     };
     services.postgresql = {
@@ -73,6 +74,7 @@ in
             wal_level = "logical";
             wal_receiver_timeout = "60s";
             max_wal_senders = 16;
+            # Should be override by postgreqql.auto.conf generated by pg_basebackup
             primary_conninfo = "host=${masterIP} port=5432 user=replication";
             hot_standby = "on";
         };