In pg_upgade, document that the port numbers must be different when

checking an old running server.

doc/src/sgml/pgupgrade.sgml

@@ -367,8 +367,10 @@ pg_upgrade.exe
      Obviously, no one should be accessing the clusters during the
      upgrade.  <application>pg_upgrade</> defaults to running servers
      on port 50432 to avoid unintended client connections.
-     You can use the same port numbers for both clusters because the
-     old and new clusters will not be running at the same time.
+     You can use the same port number for both clusters when doing an
+     upgrade because the old and new clusters will not be running at the
+     same time.  However, when checking an old running server, the old
+     and new port numbers must be different.
     </para>
 
     <para>