pg_upgrade doc: mention that replication slots must be recreated

Reported-by: Nikhil Shetty

Discussion: https://postgr.es/m/CAFpL5Vxastip0Jei-K-=7cKXTg=5sahSe5g=om=x68NOX8+PUA@mail.gmail.com

Backpatch-through: 10

doc/src/sgml/ref/pgupgrade.sgml

@@ -603,7 +603,8 @@ 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_start_backup()</function> and <function>pg_stop_backup()</function>
        or take a file system backup as the standbys are still synchronized
-       with the primary.)
+       with the primary.)  Replication slots are not copied and must
+       be recreated.
       </para>
      </step>