doc: "Latest checkpoint location" will not match in pg_upgrade

Mention that "Latest checkpoint location" will not match in pg_upgrade
if the standby server is still running during the upgrade, which is
possible.  "Match" text first appeared in PG 9.5.

Reported-by: Paul Bonaud

Discussion: https://postgr.es/m/c7268794-edb4-1772-3bfd-04c54585c24e@trainline.com

Backpatch-through: 9.5

doc/src/sgml/ref/pgupgrade.sgml

@@ -326,7 +326,8 @@ NET STOP postgresql-&majorversion;
      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.)  Also, change <varname>wal_level</varname> to
+     before the old primary or if the old standby servers are still running.)
+     Also, change <varname>wal_level</varname> to
      <literal>replica</literal> in the <filename>postgresql.conf</filename> file on the
      new primary cluster.
     </para>