pg_upgrade: mention the need for tablespace snapshots in docs

doc/src/sgml/pgupgrade.sgml

@@ -570,9 +570,10 @@ psql --username postgres --file script.sql postgres
    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">.  If your file system supports
-   file system snapshots or copy-on-write file copying, you can use that
-   to make a backup of the old cluster, though the snapshot and copies
-   must be created simultaneously or while the database server is down.
+   file system snapshots or copy-on-write file copies, you can use that
+   to make a backup of the old cluster and tablespaces, though the snapshot
+   and copies must be created simultaneously or while the database server
+   is down.
   </para>
 
  <refsect2>