Document that the pg_upgrade user of rsync might want to skip some

files, like postmaster.pid.

Backpatch to 9.2.

doc/src/sgml/pgupgrade.sgml

@@ -530,7 +530,9 @@ psql --username postgres --file script.sql postgres
    of the old cluster, use <command>rsync</> to create a dirty
    copy of the old cluster while the server is running, then shut down
    the old server and run <command>rsync</> again to update the copy with any
-   changes to make it consistent.
+   changes to make it consistent.  You might want to exclude some
+   files, e.g. <filename>postmaster.pid</>, as documented in <xref
+   linkend="backup-lowlevel-base-backup">.
   </para>
 
  <refsect2>