Doc: Improve upgrade for streaming replication section.

Currently the documentation of upgrade for streaming replication section
says that logical replication slots will be copied, but the logical
replication slots are copied only if the old primary is version 17.0 or
later.

Author: Shubham Khanna
Discussion: https://postgr.es/m/CAHv8RjJHCw0jpUo9PZxjcguzGt3j2W1_NH=QuREoN0nYiVdVeA@mail.gmail.com

doc/src/sgml/ref/pgupgrade.sgml

@@ -767,9 +767,12 @@ rsync --archive --delete --hard-links --size-only --no-inc-recursive /vol1/pg_tb
        Configure the servers for log shipping.  (You do not need to run
        <function>pg_backup_start()</function> and <function>pg_backup_stop()</function>
        or take a file system backup as the standbys are still synchronized
-       with the primary.)  Only logical slots on the primary are copied to the
-       new standby, but other slots on the old standby are not copied so must
-       be recreated manually.
+       with the primary.) If the old primary is prior to version 17.0, then no
+       slots on the primary are copied to the new standby, so all the slots on
+       the old standby must be recreated manually. If the old primary is
+       version 17.0 or later, then only logical slots on the primary are copied
+       to the new standby, but other slots on the old standby are not copied,
+       so must be recreated manually.
       </para>
      </step>