Mention dbsize in database sizing manual section.

doc/src/sgml/diskusage.sgml

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 <chapter id="diskusage">
@@ -85,6 +85,11 @@ play-# ORDER BY relpages DESC;
 </programlisting>
   </para>
 
+  <para>
+   <application>dbsize</> loads functions into your database that allow
+   you to find the size of a table or database from inside
+   <application>psql</> without the need for <command>VACUUM/ANALYZE.</>
+  </para>
   <para>
    You can also use <application>oid2name</> to show disk usage. See
    <filename>README.oid2name</> for examples. It includes a script