Simplify the example of VACUUM in documentation.

Previously a detailed activity report by VACUUM VERBOSE ANALYZE was
described as an example of VACUUM in docs. But it had been obsolete
for a long time. For example, commit feb4f44
updated the content of that activity report in 2003, but we had
forgotten to update the example.

So basically we need to update the example. But since no one cared
about the details of VACUUM output and complained about that mistake
for such long time, per discussion on hackers, we decided to get rid
of the detailed activity report from the example and simplify it.

Back-patch to all supported versions.

Reported by Masahiko Sawada, patch by me.
Discussion: https://postgr.es/m/CAD21AoAGA2pB3p-CWmTkxBsbkZS1bcDGBLcYVcvcDxspG_XAfA@mail.gmail.com

doc/src/sgml/ref/vacuum.sgml

@@ -225,38 +225,11 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">
   <title>Examples</title>
 
   <para>
-   The following is an example from running <command>VACUUM</command> on a
-   table in the regression database:
+   To clean a single table <literal>onek</literal>, analyze it for
+   the optimizer and print a detailed vacuum activity report:
 
 <programlisting>
-regression=# VACUUM (VERBOSE, ANALYZE) onek;
-INFO:  vacuuming "public.onek"
-INFO:  index "onek_unique1" now contains 1000 tuples in 14 pages
-DETAIL:  3000 index tuples were removed.
-0 index pages have been deleted, 0 are currently reusable.
-CPU 0.01s/0.08u sec elapsed 0.18 sec.
-INFO:  index "onek_unique2" now contains 1000 tuples in 16 pages
-DETAIL:  3000 index tuples were removed.
-0 index pages have been deleted, 0 are currently reusable.
-CPU 0.00s/0.07u sec elapsed 0.23 sec.
-INFO:  index "onek_hundred" now contains 1000 tuples in 13 pages
-DETAIL:  3000 index tuples were removed.
-0 index pages have been deleted, 0 are currently reusable.
-CPU 0.01s/0.08u sec elapsed 0.17 sec.
-INFO:  index "onek_stringu1" now contains 1000 tuples in 48 pages
-DETAIL:  3000 index tuples were removed.
-0 index pages have been deleted, 0 are currently reusable.
-CPU 0.01s/0.09u sec elapsed 0.59 sec.
-INFO:  "onek": removed 3000 tuples in 108 pages
-DETAIL:  CPU 0.01s/0.06u sec elapsed 0.07 sec.
-INFO:  "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages
-DETAIL:  0 dead tuples cannot be removed yet.
-There were 0 unused item pointers.
-0 pages are entirely empty.
-CPU 0.07s/0.39u sec elapsed 1.56 sec.
-INFO:  analyzing "public.onek"
-INFO:  "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows
-VACUUM
+VACUUM (VERBOSE, ANALYZE) onek;
 </programlisting></para>
  </refsect1>