Doc: Add a note in the upgrade of logical replication clusters.

The steps used to upgrade the cluster first upgraded the publisher node
but ideally, any node could be upgraded first.

Author: Vignesh C
Discussion: https://postgr.es/m/CALDaNm1_iDO6srWzntqTr0ZDVkk2whVhNKEWAvtgZBfSmuBeZQ@mail.gmail.com
Discussion: https://postgr.es/m/CALDaNm3Y-M+kAqr_mf=_C1kNwAB-cS6S5hTHnKMEqDw4sGEh4Q@mail.gmail.com

doc/src/sgml/logical-replication.sgml

@@ -2564,6 +2564,14 @@ ALTER SUBSCRIPTION
        </para>
       </step>
      </procedure>
+
+     <note>
+      <para>
+       In the steps described above, the publisher is upgraded first, followed
+       by the subscriber. Alternatively, the user can use similar steps to
+       upgrade the subscriber first, followed by the publisher.
+      </para>
+     </note>
     </sect3>
 
     <sect3 id="steps-cascaded-logical-replication-cluster">