doc: PG 13 relnotes, add pgbench script item

Author: bmomjian

doc/src/sgml/release-13.sgml

@@ -2108,6 +2108,18 @@ Allow pgbench to generate its data server-side, rather than client side (Fabien
 DOCUMENT THE DEFAULT GENERATION METHOD
 </para>
 
+</listitem>
+
+<listitem>
+<!--
+Author: Thomas Munro <[email protected]>
+2019-07-16 [ce8f94676] Report the time taken by pgbench initialization steps.
+-->
+
+<para>
+Allow pgbench to dump script contents using --show-script (Fabien Coelho)
+</para>
+
 </listitem>
 
      </itemizedlist>