doc: PG 16 relnotes, add memory overhead reduction item

Reported-by: David Rowley

Discussion: https://postgr.es/m/CAApHDvqnUoyrJxS4uqnKo87b3_cUjaDDte4NQag49YHu1oEOgA@mail.gmail.com

Author: bmomjian

doc/src/sgml/release-16.sgml

@@ -2798,6 +2798,17 @@ Author: Peter Eisentraut <peter@eisentraut.org>
 <para>
 Adjust pg_attribute columns for efficiency (Peter Eisentraut)
 </para>
+</listitem>
+
+<!--
+Author: David Rowley <drowley@postgresql.org>
+2022-08-29 [c6e0fe1f2] Improve performance of and reduce overheads of memory ma
+-->
+
+<listitem>
+<para>
+Reduce overhead of memory allocations (Andres Freund, David Rowley)
+</para>
 </listitem>
 
     </itemizedlist>