doc: pg_upgrade, clarify standby servers must remain running

Also mention that mismatching primary/standby LSNs should never
happen.

Reported-by: Nikolay Samokhvalov

Discussion: https://postgr.es/m/CAM527d8heqkjG5VrvjU3Xjsqxg41ufUyabD9QZccdAxnpbRH-Q@mail.gmail.com

Backpatch-through: 11

doc/src/sgml/ref/pgupgrade.sgml

@@ -325,8 +325,8 @@ NET STOP postgresql-&majorversion;
     </para>
 
     <para>
-     Streaming replication and log-shipping standby servers can
-     remain running until a later step.
+     Streaming replication and log-shipping standby servers must be
+     running during this shutdown so they receive all changes.
     </para>
    </step>
 
@@ -339,8 +339,6 @@ NET STOP postgresql-&majorversion;
      servers are caught up by running <application>pg_controldata</application>
      against the old primary and standby clusters.  Verify that the
      <quote>Latest checkpoint location</quote> values match in all clusters.
-     (There will be a mismatch if old standby servers were shut down
-     before the old primary or if the old standby servers are still running.)
      Also, make sure <varname>wal_level</varname> is not set to 
      <literal>minimal</literal> in the <filename>postgresql.conf</filename> file on the
      new primary cluster.