Adjust documentation for vacuumdb --disable-page-skipping

This makes the description more consistent with the other options, and
the mapping with VACUUM is intuitive.

Author: Nathan Bossart
Discussion: https://postgr.es/m/FFE5373C-E26A-495B-B5C8-911EC4A41C5E@amazon.com

doc/src/sgml/ref/vacuumdb.sgml

@@ -106,9 +106,7 @@ PostgreSQL documentation
       <term><option>--disable-page-skipping</option></term>
       <listitem>
        <para>
-        Disable all page-skipping behavior during processing based on
-        the visibility map, similarly to the option
-        <literal>DISABLE_PAGE_SKIPPING</literal> for <command>VACUUM</command>.
+        Disable skipping pages based on the contents of the visibility map.
        </para>
        <note>
         <para>