doc: Improve pg_upgrade rsync examples to use clusterdir

Commit 9521ce4a from Sep 13, 2017 and
backpatched through 9.5 used rsync examples with datadir.  The reporter
has pointed out, and testing has verified, that clusterdir must be used,
so update the docs accordingly.

Reported-by: Don Seiler

Discussion: https://postgr.es/m/CAHJZqBD0u9dCERpYzK6BkRv=663AmH==DFJpVC=M4Xg_rq2=CQ@mail.gmail.com

Backpatch-through: 9.5

doc/src/sgml/ref/pgupgrade.sgml

@@ -494,10 +494,10 @@ pg_upgrade.exe
        server:
 
 <programlisting>
-rsync --archive --delete --hard-links --size-only --no-inc-recursive old_pgdata new_pgdata remote_dir
+rsync --archive --delete --hard-links --size-only --no-inc-recursive old_cluster new_cluster remote_dir
 </programlisting>
 
-       where <option>old_pgdata</option> and <option>new_pgdata</option> are relative
+       where <option>old_cluster</option> and <option>new_cluster</option> are relative
        to the current directory on the primary, and <option>remote_dir</option>
        is <emphasis>above</emphasis> the old and new cluster directories
        on the standby.  The directory structure under the specified
@@ -506,8 +506,8 @@ rsync --archive --delete --hard-links --size-only --no-inc-recursive old_pgdata
        remote directory, e.g.
 
 <programlisting>
-rsync --archive --delete --hard-links --size-only --no-inc-recursive /opt/PostgreSQL/9.5/data \
-      /opt/PostgreSQL/9.6/data standby.example.com:/opt/PostgreSQL
+rsync --archive --delete --hard-links --size-only --no-inc-recursive /opt/PostgreSQL/9.5 \
+      /opt/PostgreSQL/9.6 standby.example.com:/opt/PostgreSQL
 </programlisting>
 
        You can verify what the command will do using