docs: PG 12 relnotes, move vacuumdb to the client app section

It was previously incorrectly placed in the server application section.

Reported-by: Tatsuo Ishii

Discussion: https://postgr.es/m/20190526.215341.1023150119477784132.t-ishii@sraoss.co.jp

doc/src/sgml/release-12.sgml

@@ -2442,6 +2442,23 @@ Author: Peter Eisentraut <peter_e@gmx.net>
 
      <listitem>
 <!--
+Author: Michael Paquier <michael@paquier.xyz>
+2019-01-31 [00d1e88d3] Add - -min-xid-age and - -min-mxid-age options to vacuumdb
+-->
+
+      <para>
+       Allow <xref linkend="app-vacuumdb"/> to select tables for vacuum
+       based on their wraparound horizon (Nathan Bossart)
+      </para>
+
+      <para>
+       The options are <option>--min-xid-age</option> and
+       <option>--min-mxid-age</option>.
+      </para>
+     </listitem>
+
+     <listitem>
+<!--
 Author: Peter Eisentraut <peter@eisentraut.org>
 2019-04-01 [cc8d41511] Unified logging system for command-line programs
 -->
@@ -2602,23 +2619,6 @@ Author: Peter Eisentraut <peter_e@gmx.net>
      <listitem>
 <!--
 Author: Michael Paquier <michael@paquier.xyz>
-2019-01-31 [00d1e88d3] Add - -min-xid-age and - -min-mxid-age options to vacuumdb
--->
-
-      <para>
-       Allow <xref linkend="app-vacuumdb"/> to select tables for vacuum
-       based on their wraparound horizon (Nathan Bossart)
-      </para>
-
-      <para>
-       The options are <option>--min-xid-age</option> and
-       <option>--min-mxid-age</option>.
-      </para>
-     </listitem>
-
-     <listitem>
-<!--
-Author: Michael Paquier <michael@paquier.xyz>
 2019-01-08 [354e95d1f] Add - -disable-page-skipping and - -skip-locked to vacuumd
 -->