doc: add PG 10 doc item about VACUUM truncation, 7e26e02e

Reported-by: Andres Freund

doc/src/sgml/release-10.sgml

@@ -1744,6 +1744,22 @@
       </para>
      </listitem>
 
+     <listitem>
+      <!--
+      Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
+      2017-01-23 [7e26e02ee] Prefetch blocks during lazy vacuum's truncation scan
+      -->
+      <para>
+       Improve speed of <command>VACUUM</>'s removal of trailing empty
+       heap pages (Alvaro Herrera)
+      </para>
+
+      <para>
+       This information is also included in <xref
+       linkend="guc-log-autovacuum-min-duration"> output.
+      </para>
+     </listitem>
+
      <listitem>
       <!--
       Author: Tom Lane <tgl@sss.pgh.pa.us>