Document that Log-Shipping Standby Servers cannot be upgraded by

pg_upgrade.

Backpatch to 9.2.

doc/src/sgml/pgupgrade.sgml

@@ -517,6 +517,12 @@ psql --username postgres --file script.sql postgres
    <literal>-d /real-data-directory -o '-D /configuration-directory'</>.
   </para>
 
+  <para>
+   A Log-Shipping Standby Server (<xref linkend="warm-standby">) cannot
+   be upgraded because the server must allow writes.  The simplest way
+   is to upgrade the primary and use rsync to rebuild the standbys.
+  </para>
+
   <para>
    If you want to use link mode and you do not want your old cluster
    to be modified when the new cluster is started, make a copy of the